Here is how I see it. I may not be perfectly accurate. The bank is LICENSED to AUTHENTICATE Federal Reserve Notes. They are licensed and audited by their regional bank. If they fail the audit they are bankrupt. Just like any corporation a bank can lose its sanction to operate if their balance sheet is in the red. Yes, they can lose from loan defaults. They must account for the FRNs that they AUTHENTICATED in accordance with their LICENSE. The bank is a franchise of the federal reserve system.