Exchange rates fluctuate constantly. If on the date you received the payment, the exchange rate was lower than that of the invoice, the value of the payment in the home currency will result to more than the actual invoice amount. When allocating the payment, an amount will be outstanding in the home currency, where, you will have to process a Forex Movement to settle the payment.
Navigate to Sales > Invoices and click on the Create invoice button.
Select Forex Movement in the Account drop down list. Make sure to use the same date as the original payment and use the same customer as the original invoice. The unit price should be for the remaining unallocated amount of the receipt (using the same currency as the receipt).
Once the invoice has been confirmed, hit the "Allocate" button.
Select the currency (it should default to the home currency) and select the appropriate payment (if you have more than one). Type in the amount to be allocated to the invoice in the 'Allocate payment' box and hit the 'Allocate' button to complete.