Multi Currency - Exchange Gain/Loss

I have an issue. We trade in multi-currency. We purchase in USD and make the payment from our bank in AED.

When I receive an invoice for USD 1,000 the system considers it as AED 3,685 because of the exchange rate set. Now when I make the payment, the bank has its own exchange rate. So for this payment, if the bank has deducted AED 3,680, that gets accounted as Exchange Gain of AED 5.

The issue is if I make any advance payment to the supplier and if I enter the invoice later, I can go ahead and allocate supplier payment, but there is no way to tell the software that it is full and final payment. The invoice stays as overdue because of the AED 5.

Am I doing something wrong? Is there a way I can enter a balancing Exchange Gain of AED 5 to clear the invoice?