Click on Purchase Ledger.
Click on Pay Supplier.
Click on Firm Pre-Payment - Make sure the Firm Invoices box is highlighted on the right-hand side.
Enter the amount of the overpayment.
Post.
Click on Purchase ledger.
Click Pay Supplier.
Enter the Supplier reference number.
Leave the Amount '0.00'.
Tick the Prepayment amount from above and the Purchase Invoice Amount.
Post.