Normally, financial transactions are done on the street. If I'm a drug dealer and three inmates each owe me $100, I'll provide those three inmates with a bank account number and tell inmate number 1 to deposit $100.06 into that specific bank account. I will tell inmate number 2 to deposit $100.12 into that specific bank account, and inmate number 3, maybe $100.16. Then I'll phone my contact on the street to check the deposits into that account. If I know there is $100.06 deposited in that account, inmate number 1 has paid his debt.
Most of the financial transactions are not done inside the institution but on the street.