Script wise, we have bank accounts, and we have player id's this shouldn't be too hard.
Player 1 with ID: 1234
Player 2 with ID: 4321
Both of these player id's are different, much like in game, so based upon player id's you could also assign each player a randomly generated Account Number, with a default Routing because theres only one bank...
So you get....
Player 1 with ID: 1234 - Account Number 1011
Player 2 with ID: 4321- Account Number 1100
Player 1 now owes Player 2 5k, but Player 2 is not in town or busy somewhere else.
Player 1 goes to the bank and fills in a small information tab, much like collecting salary or sending a text /wiretransfer or /wt - (account #) (message) (amount)
Sending to Account Number: 1100
Message: Loan Debt
Amount: $5,000
Player 2 Receives a message on his phone or bank statement, which shows the information the sender entered.
Done, no further contact required.
That's it, wire transfers, pretty simple.
This was not my suggestion, but a good one so I made a suggestion for it.