Can I reverse a write off that already appeared on an invoice?

A client owed $100.40 but paid $100.00, so we wrote off the 40¢. We sent a $50 invoice the next month which showed the write off. The client paid $50.40 after he saw the write off. Is there a way to reverse the write off so I can apply the new full payment? If there is no way to reverse the write off, what is the best way to handle the new payment?