Customer underpayments - no solution?

We have a huge problem with customer underpayments. I have been talking to support for three hours this morning and seemingly their is no solution. I cannot believe we are alone with this problem and I would like to know how you have over come this problem:-

Example We have a customer with two outstanding invoices:-

INV1212 £200.00
INV1302 £150.00

Total outstanding £350.00

Remittance advice received from customer:-

INV1212 £200.00
INV1302 £150.00
DN1103 £  80.00

Total Payment £270.00

The customer has deducted £80.00 in respect of an old invoice paid two months ago. On the customers account we must show that 1212 & 1302 are paid in full and an underpayment of £80.00, we need this £80.00 to appear on the customer statement so that we can pursue him for the money.