The special provision in the Uniform Commercial Code (section 3-312) that provides the purchaser/remitter or payee the right to make a Lost/Stolen/Destroyed claim on a cashier's, teller's, or certified check has no provision for "buyer's remorse." So don't even think you can stop payment after 90 days. As Brian said, there is nothing the bank can do to restore its customer's funds.
Given the dollar amount -- $50,000 -- the bank should be considering how it will be completing its SAR filing.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8