To apply a prepayment or other payment documents to this invoice received, click 
(the Edit prepayment offset button) to the right of the Prepayment field at the bottom of the form. This opens the Offset Prepayment dialog box to be used for selecting payments made earlier to the supplier.
The "Invoice Received (create)" form. The Offset Prepayments dialog box
The dialog box contains two panes:
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To automatically apply the payments to the invoice received, click the Auto Fill button. The system selects the documents starting from the earliest ones (by using the first-in first-out rule) until either all the available documents are applied or the amount of the applied documents becomes equal to the invoice amount.
To manually apply a prepayment, select a prepayment in the upper pane and double-click it. Repeat for other prepayments if needed. Use the Up and Down arrows in the group 
of arrows to rearrange the documents in the lower pane. Then, in the Amount column, you can edit the amounts of the payments that must be applied—the remaining amounts in the upper pane will be adjusted accordingly.
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