I am trying to understand what the actual issue is here. Is it loss prevention? (If so, is there an actual history of loss here?) Do you keep a ledger of monies received vs forgiven? Or is it that those in charge of collecting funds are arbitrarily forgiving debt?
I can't really speak to best practices from my library, but in my previous career I managed a bank and you can feel free to email me if you need some advice on putting cash controls in place. I agree that whatever solution you come to shouldn't impact your patrons.
Steven Kish