If the money is to mitigate risk, then why can't the fee be (mostly) refundable? The bank takes on the risk of processing some enormous transaction and takes a

If the money is to mitigate risk, then why can't the fee be (mostly) refundable? The bank takes on the risk of processing some enormous transaction and takes a percentages of the transaction as insurance in case of fraud. Then after a period of time with no issues, the money flows back, less some nominal amount for the actual work involved processing the transaction.