Dunning letter

We are on R12 and when printing dunning letter we are having an issue:

Scenario:

1. A customer has an outstanding invoice which is overdue and should be on the dunning letter with late charges.

2. Sometimes we get advance payments from customers and enter in the system as receipts On Account as we do not have an invoice to apply.

If we run the dunning at this instance the system takes into account this receipt and does not generate late charges and dunning if the total of 1 and 2 is zero OR just creates dunning for the balance amount.

We dont want the system to count the On account receipts towards late charge / dunning generation.