Partial Payment for an outstanding Invoice

Good Morning.  New to SL so please bear with me if this is a basic question. ....  Invoice 203600 was  entered and sent to the customer for $270,000.00.  We received payment in January for $250,000.00.  I applied the $250k using "Payment Application".  The remaining $20k has now come in, but I can't apply it to this invoice as it and I get the error "This record exists, therefore it cannot be added at this time".  How do I apply the remaining $20k to Invoice 203600 so it doesn't appear on the "Aged SR - Detail by Days Past Due" Report?

Thank  You