...
To automatically apply the prepayments to the invoice, 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 documents 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 amounts in the upper pane will be adjusted accordingly.
...