After a few years of trying to collect on an invoice from 2015, I received permission from my boss to write off the bad debt in 2019. Today, out of the blue, the customer made a payment for the full amount of the original invoice. Our company accountant has closed our books through 2019. How do I undo the old write off and apply today's payment (2020) to the old invoice without altering the books from previous years?

TYIA for your help!