In order to save your time, I've decided to put the explanation, QnA and troubleshoting related sections after the main instructions.
For starters - you just need to understand which INSTRUCTION to choose and follow this INSTRUCTION.

• don't want to / are tired of being robbed by Bandits but want to stay neutral with them;
  
• don't want to / are tired of being robbed during the mission "Catch up with Fang" ("Flea Market" basement on the "Garbage" location);
  
• forgot how to do the "Money Glitch";
  
• tried the methods from the internet but they didn't work;
  
• did the "Money Glitch" sometime ago, it worked but now it doesn't work;
  
• want to get money, but "don't want to cheat" ("game exploit is OK, but external applications such as ArtMoney are NOT OK");
  
• want to understand the nature and backend of the "Money Glitches".

The INSTRUCTIONS describe the same "Money Glitch" used but in 2 different situations. Depending on your state of the game and your intentions, both of the INSTRUCTIONS or only 1 of them will be suitable for you. Read the information below to be able to understand which INSTRUCTION you should follow.

1. you just started the game (or later)
   
2. ...and you have a "neutral" or "positive" reputation with Bandits
   
3. ...and you didn't finish the Bandits' mission after which they let you pass freely and don't rob you.

NOTE 1: this method can be repeated as long as #2 and #3 don't change, but will not be able to get more than 2 billion RU.
NOTE 2: if either #2 or #3 have changed AND you haven't completed the main mission that requires you to follow the Fang's PDA into the basement of the Flea Market on the "Garbage" location - switch to "[INSTRUCTION 2]".

• you are ready to complete the main mission that requires you to follow the Fang's PDA into the basement of the Flea Market on the "Garbage" location

NOTE: this method cannot be repeated. If the Bandits are hostile to you OR you have completed the Bandits' mission that allows you to pass freely without getting robbed - this is the last chance to do the Money Glitch!!!

Here is a video demonstration of the main steps for this instruction:
Link to the video, in case there are problems with the YouTube player: https://youtu.be/IkMYyUSkYU0

1. Follow the main quest until you get to the "Cordon" location (in any way).
2. Make sure you have the "PMm" pistol that is not fully upgraded (important).

• if the "PMm" is fully upgraded - this method will not work with this pistol, another one is needed
  
• if you don't have it in your inventory - get it in any available way
  
• even if it's your "main" pistol and its modified - don't worry, you can use it too (+ you can save in step #3)

3. Make a Hard Save from the Main Menu and name it something you will remember (i.e. "before money glitch preparation").
4. Drop the "PMm" pistol on the ground.

• it's better to do just outside the "Neutral Base"

5. Damage the dropped "PMm" until its condition becomes "0".

• 1-2 hits with the knife should be enough

6. Don't forget to pick up the dagamed "PMm".
7. (optional) You can repeat steps #4-6 with any of the weapons or suits that you like - in a few moments you will be able to repair and modify them for free!
8. Go to any nearest Trader[stalker.fandom.com].
9. Trade with him until you have between "121" and "239" RU in your inventory.

• yes, will most likely need to purchase something expensive - its okey for now :)

10. Go to the nearest Technician[stalker.fandom.com] and get to the "Upgrade" screen.
11. Click once on your "PMm".
12. THIS IS IMPORTANT! First, click on the icon with cogwheels (like you want to repair the "PMm").

• if this step is skipped - you will need to restart from step #8

13. Click "OK".
14. Buy at least one upgrade for this "PMm" while not clicking on any other weapon/suit or closing the "Upgrade" screen (important).

• if you clicked on any other weapon/suit or closed the "Upgrade" screen - close the "Upgrade" screen and repeat from step #10

15. You should now have a negative amount of money (i.e. "-50RU"). If not - you did something wrong and have to restart from the Save or refer to the "Troubleshooting" section.
16. (optional) You can now fully repair and upgrade all of your weapons/suits for free!

• the more negative amount of money you have - the more money you will get at the end (but it won't stack up as much as you would hope for - see "QnA")

17. Go to the passage between the "Cordon" and the "Garbage" locations that leads to the Bandits outpost where they usually rob you.
18. Cross to the "Garbage" location and wait until it loads, stay in the tube and don't go to the Bandits yet.
19. Make a Hard Save from the Main Menu and name it something you will remember (i.e. "just before the money glitch with bandits").
20. Now go to the Bandits and do all that they are asking (lower the weapon, agree to pay them).

• load the Save from #19 if something didn't go as planned

21. You should now have a bit less than "2 147 483 648+ RU" in your inventory.

If you want the Bandits' reputation to stay "Neutral" or "Positive":

• avoid the Bandit posts where you can be robbed until you complete the Bandits' mission which allows you to pass these outposts freely

If you don't care about the Bandits:

• just kill them

If you are ready to complete the main mission where you have to follow the Fang's PDA into the Flea Market basement on the "Garbage" location:

• follow the [INSTUCTION 2]

Here is a video demonstration of the main steps for this instruction:
Link to the video, in case there are problems with the YouTube player: https://youtu.be/0zHe8FjuiQM

1. Follow the main quest until you get to the "Catch up with Fang" mission.
2. Make sure you have the "PMm" pistol that is not fully upgraded (important).

• if the "PMm" is fully upgraded - this method will not work with this pistol, another one is needed
  
• if you don't have it in your inventory - get it in any available way
  
• even if it's your "main" pistol and its modified - don't worry, you can use it too (+ you can save in step #3)

3. Make a Hard Save from the Main Menu and name it something you will remember (i.e. "before fang pda money glitch preparation").
4. Drop the "PMm" pistol on the ground.

• it's faster to on the "Flea Market"

5. Damage the dropped "PMm" until its condition becomes "0".

• 1-2 hits with the knife should be enough

6. Don't forget to pick up the dagamed "PMm".
7. (optional) You can repeat steps #4-6 with any of the weapons or suits that you like - in a few moments you will be able to repair and modify them for free!
8. Go to any nearest Trader[stalker.fandom.com].
9. Trade with him until you have between "121" and "239" RU in your inventory.

• yes, will most likely need to purchase something expensive - its okey for now :)

10. Go to the nearest Technician[stalker.fandom.com] and get to the "Upgrade" screen.
11. Click once on your "PMm".
12. THIS IS IMPORTANT! First, click on the icon with cogwheels (like you want to repair the "PMm").

• if this step is skipped - you will need to restart from step #8

13. Click "OK".
14. Buy at least one upgrade for this "PMm" while not clicking on any other weapon/suit or closing the "Upgrade" screen (important).

• if you clicked on any other weapon/suit or closed the "Upgrade" screen - close the "Upgrade" screen and repeat from step #10

15. You should now have a negative amount of money (i.e. "-50RU"). If not - you did something wrong and have to restart from the Save or refer to the "Troubleshooting" section.
16. (optional) You can now fully repair and upgrade all of your weapons/suits for free!

• the more negative amount of money you have - the more money you will get at the end (but it won't stack up as much as you would hope for - see "QnA")

17. Make a Hard Save from the Main Menu and name it something you will remember (i.e. "just before the fang pda money glitch").
18. If you don't want to loose all of your items - stash them somewhere prior to entering the basement.
19. Now follow the main mission objective and go to the basement of the Flea Market at the "Garbage" location. Get flashbanged and looted.

• load the Save from #17 if something didn't go as planned

20. You should now have a bit less than "2 147 483 648+ RU" in your inventory.

If you want the Bandits' reputation to stay "Neutral" or "Positive":

• avoid the Bandit posts where you can be robbed until you complete the Bandits' mission which allows you to pass these outposts freely

If you don't care about the Bandits:

• just kill them

After reading the Instructions you should already have a rough understanding on how the "Money Glitches" work. Unfortunatelly, I didn't get access to the code and I don't know all of the specifics from inside (+ this guide is already too long for any additional info + I am lazy). This is all I've gathered during my research on the internet + testing in game.

Part 1 - "negative" money
With some manipulations, the player can get to the "negative" amounts of money. This leads to some interesting gameplay features.

Positive aspects:

• all available travels are unlocked (doesn't depend on the money you have) and free
  
• weapon/suit repairs are free
  
• weapon/suit upgrades are free

Note^: by "free" I mean that you can do as much as you want, but the "negative" money balance will increase towards more "negative" values (it doesn't really affect anything).

Negative aspects:

• the player doesn't have the ability to trade (TODO - reasearch the specifics)

Undescovered aspects:

• TODO - research how the money rewards affect the "negative" money balance (probably decrease towards more "positive")

Part 2 - robbed "negative" money turn into "positive" billions
During the game, there are a few cases where the player is getting robbed off of his belongings and his money. As far as I know, this is done on some of the Bandits' outposts and during the mission where you have to follow the Fang's PDA.
Simpified algorithm for the money exchange during the robbery is as follows:

1. *do some broken code* that should take away the player's money
   
2. *do some broken code* that should set the player's money to "0 RU"

The code works fine for the "positive" values but is completely broken for the "negative".
Theoretically, the maximum amount that can be given to the player at the end is "2 147 483 648 RU". In practive it will be lower by the "negative" value you had prior to being robbed.

Part 1 - "negative" money
Personally, I know only one specific way to get to the "negative" amounts of money: "try to repair when you don't have enough money->upgrade the weapon/suit with enough money for the upgrade but not enough for the price of the repair".
In the instructions I used a very specific example with "PMm" for the ease of understanding, but theoretically it can be done with any weapon/suit when these general steps are followed (not thoroughly tested by me):

1. get the weapon/suit with condition <100% and at least 1 upgrade that is not yet installed
   
2. make sure the player has more money than the cheapest available upgrade but less than the price of the repair
   
3. try to repair
   
   • on the UI all seems to work fine (it says that you don't have enogh money for the repair), but on the backend for some reason it seems to store *something* that later allows the glitch to happen. This *something* - is either a 100% price value of the repair or 50% price value of the repair.
4. without switching to another weapoin/suit or closing the "Upgrade" screen - purchase the upgrade for the same weapon/suite
   
   • if I am not mistaken - after switching to another weapon/suit or closing the "Upgrade" screen, the stored value is being cleared. Not sure though.
     
   • during this step the full price of the repair (?) is being subtracted from the players money balance, thus making it into a negative value.

I've tested only with weapons with condition at 0%, so my conclusions might be incorrect. Lets review a specific example with "PMm":

• starting condition of the "PMm" is at 0%
  
• I have "124RU", cost of repair is "240RU"
  
• any first tier upgrade is "120RU"
  
• after the glitch is done I end up with "-116RU", so it just subtracted "240RU". Is a 100% repair price, a 50% repair price + upgrade price, or a doubled upgrade price - I have no idea :C I have to do more research.

Part 2 - robbed "negative" money turn into "positive" billions
Here I can only assume that the issue is with data type transfer and the way it is implemented.
The money that is taken from us is near "4 294 967 296" which is 2^32 (minus the value of the players money balance), but the money value that we receive back is near "2 147 483 648" which is 2^31 or (2^32)/2 (minus the value of the players money balance).
Why doesn't it just assign a "0" value instead of some shenanigans with transfers and subtractions - I have no idea (I am not a programmer, nor a game developer).

Q1: Is the "Money Glitch" in any way better at getting money than trainers or "ArtMoney" (or whatever its called)?
A1: The "Money Glitch" is way less efficient and way more complex to execute but doesn't require any external intervention. The external programs are more efficient and simple to use (build specifically for this task), but may contain viruses. One is considered an "exploit", others are considered as "cheats". I will leave this "highly philosophical" question up to you :)

Q2: Do I have to do this only with "PMm"?
A2: No, but its easier with PMm. You can do this with any weapon/suit (not all upgrades purchased) that can be purchased at your Technician. To do this - lower the condition to 0%, check how much the repair will cost and make sure your money balance is more than the price for the upgrade and less than the price of the repair.

Q3: Do I have to go only to the Traders and Technicians from the videos?
A3: Nope, just choose the ones that are closer to you and are not hostile. For Technicians - you can use this info[stalker.fandom.com] to see if they will be able to upgrade your weapon/suit (all Technicians can upgrade "PMm").

Q4: Can I get more than "2 147 483 648 RU" with these methods?
A4: Nope. And I think even "ArtMoney" (or whatever its called) won't give you more.

Q5: Is it safe do the "Money Glitch" with my uber gucci'ed out weapon/suit that I've put 100000+ RU?
A5: Yes! Especially if you make a Hard Save prior to lowering the condition of your item. You can perform the "Money Glitch" with your item as well as fully repair/upgrade once you've reached the "negative" money balance - the repairs and upgrades won't go anywhere until the end of the game.

Q6: What if the Bandits dont rob me anymore (hostile or they let me pass for free) and I lost money during the "Catch up with Fang" mission?
A6: "GGs". Unfortunately, I don't know any other methods. You can load an old Save if you have one or download some external trainers / cheats.

Q7: All of my money was robbed! Can I somehow get it back?
A7: Depends. Read the "How to select a suitable INSTRUCTION" section of this guide and Q6 to see if you still can do the "Money Glitch". If this won't help - load an older Save, see what internet has to say or download "ArtMoney" (or whatever its called).

Most of the specific information in this guide was gathered by me during my own troubleshooting process. I saw some "2 minute" guides/videos and tried to do the glitch with my interpretations (>0% condition, different Traders and Technicians, different weapons and suits, etc.) where it simply didn't work most of the time.

I advise you to try fixing the issue by youself before seeking help or reading the solutions below or in the comments. Maybe you will learn something useful along the way (don't forget to share it in the comments <3).

During the testing of the "Money Glitches" I've used at least 3 Hard Saves for each of the methods:

1. Just before I am about to damage weapons/suits;
   
2. Just before I am about to Trade for the correct money balance;
   
3. (optional) Just before I am about to use the Technician to get the "negative" money balance;
   
4. Just before I am about to get robbed (before talking to the Bandits or entering the basement);
   
5. (optional) Just after I've done the "Money Glitch".

Making these Saves will allow you to easily backtrack and repeat the steps or troubleshoot your problem.

P1: I am using some other weapon/suit other than "PMm" and I cannot purchase any upgrades (they are not highlighted and I cannot even click on them).
R1: Your Technician might not be able to upgrade your item. Please refer to this page[stalker.fandom.com] to see which weapons/suits your Technician can upgrade. It should be either this or you have a fully upgraded item.
S1: You can continue without loading a Save and go to another suitable Technician with the same item (if it's not filly upgraded). Otherwise - load the Save #1 and try a different item.

P2: I am not able to purchase any upgrades for my weapon/suit (I can select an upgrade, but the message is displayed saying I dont have enogh money).
R2: The error message is probably correct. You don't have enough money.
S2: Go to the nearest Trader and fix your money balance. It should be enough for the purchase of the desired upgrade but not enough for the repair. For example, for the "PMm" the first tier upgrades each have a price of "120RU" and the full repair is "240RU" (if the condition is at 0%) - that means your money balance should be between "120RU" and "239RU".

P3: I was able to upgrade the weapon/suit, but I didn't get "negative" amount of money.
R3: Most probably, you just skipped a step where you need to click on the "Repair" (cogwheels) > "OK" before upgrading your weapon/suit OR switched the selected item / closed the "Upgrade" screen.
S3: If the item is not yet fully upgraded - go back to the Trader, fix your money balance how it's described in the instructions or in S2 and continue with instructions. If the item is fully upgraded - load Save #2 or #3 and repeat. Do not forget about the "Repair" before the "Upgrade". Do not forget to not switch weapons / close "Upgrade" screen during this.

P4: The Bandits do not appear / do not rob me / shoot me.
R4: Do not appear: some sort of bug or you already killed them. Do not rob: you've probably completed a mission after which you are allowed to pass without being charged. Shoot: you have a "negative" reputation with Bandits.
S4: Reload the Save, restart the Game. If the your issue is not fixed - [INSTRUCTION 1] won't work, check if the [INSTRUCTION 2] is suitable for you and execute it.

To conclude, there is still much to research and understand about these "Money Glitches". Not all cases are yet covered.

If I will find something new or something new will find me (in the comments section, *wink-wink*) - I will update this guide.