Re: How much would it change D&D games if credit was used?
In relation to credit consider the real world medieval banking done by the Knights Templar. (Conspiracy panic earphones if you need them). Their end came when the massively indebted King of France got a friendly religious leader, the Pope, to run a religious trial and poof the King's debt was gone. This you can look up in history.
The Templars had begun escorting travelers to the Temple of Solomon. Part of their service was handling funds. Think of today's traveler's checks. A traveler starting out didn't need to carry huge amounts of coin. Coins being both a high theft risk and difficult to transport. He turned in his coin to the Templars in exchange for a document. Once at the destination, possibly escorted by the Templars, could turn over the note to the Templars for coin. One can imagine occasionally the Templars having a military convoy/ship going to move coins around as needed.
You could also imagine that the notes, a form of credit, might be traded at times of desperation or as a trade good.
Effectively the Templars, in providing safety, created an economy outside of the control of tax or tithe. Much like today's online micro-transaction or store groups that make their own 'store bucks' where you can spend their 'money' at any of their community members. Its also why the operators of Tax (King of France) and tithe (Pope) made up charges to remove them and shame them. Think of a town's mayor and pastor owing too much to a bodyguard company so wiping them out.
This is in the same time period the Pope's church was selling indulgences. That meaning a note saying your sins were gone and you were good to go into heaven because you gave cash to person in a church office. This church money work also meant they were a competitor for the same coins in people's pockets. Why take a pilgrimage when you can stay at home with the Pope's piece of paper.
Computer games like Assassian Creed really don't protray the real history of the Templars.
(conspiracy headphones in effect so please keep to relevant chatter only)
What it does show in terms of the original poster and credit is that there did exist some forms of banking but often not without the protections we have today. Credit card companies are both regulated and protected by the state. Niether applied in the traveler notes described above.
The value of the notes is only guaranteed by the providing group. Forgeries would surely be attempted. Destruction of the note might occur. There could be duplicate documents made and sent in the military convoy to verify the notes issued. An enemy religion might want to disrupt their use.
You might trade a note early. A credit note for 10 lbs silver in Jerusalem traded at Greece might get you 8 lbs silver, since the receiver needs to travel.
The above was meant to discuss one aspect of banking and credit notes from our real world which could be plausible in fantasy setting.
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In mention of credit, debt and interest it is actually a very old practice. In real life (again no prejudice or conspiracy intended just a discussion of history) Jews were often forced into banking as other trades were denied them. In the real life Christian faith one wasn't allowed to get interest on a loan but the older-religion Jews could do it. In some films like early Robin Hood or Ivanhoe it is the Jews who are the money handlers that can move the huge sums of money to pay King Richard's ransom.
Typically the matter was a double document made giving X coins to be repaired in Y days with Z additional money. Again the problem arises that it was not state regulated nor protected. However, the Kings and other big nobles themselves were customers. The big money folks protected the bankers in part because if they didn't ALL the bankers would stop dealing with them... and instead work with say the other claimants to the throne.
The loan side of the credit note could be traded. Banker 1 gives 1,000 gold to Sir Needy with Sir Needy to pay back 1,200 gold by next summer solstice. Banker 1 has a deal opportunity so trades the receiver's note to Lord East for 1,150 gold. Now Sir Needy finds he must pay Lord East 1,200 gold and Lord East will profit 50 gold.
Enter fantasy plot... maybe Sir Needy and Lord East are enemies.
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What this type of credit was though is more of a bank loan. It is not a credit card. A modern credit card allows small dollar transactions made against a big credit line. $20 grocery or $100 backpack against a $X,000 credit allowance.
What you traditionally get is a lump of cash. You then need to spend it. Possibly buying small credit notes to avoid having to carry 200 lbs of physical silver.
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So in a fantasy game you can certainly have credit in the game.
Small transaction credit cards wouldn't exist, as they take something like phone lines to check banker records to see if the allowance is there.
Loans, as an advance on credit, have certainly been around a long time.
The buying of a loan also exists, though we don't see much of it today. It would be like taking out a loan with the 1st Bank of Townsville to find you now had to pay the 3rd Bank of Nextsville.
Trading of loan ownership or credit notes was also very much a thing. It wasn't normal for most people to use though.