Exchange rates fluctuate constantly. If on the date you received the payment, the exchange rate was higher than that of the invoice, the value of the payment in the home currency will result to less 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 invoice.
Navigate Sales > Invoices.
Click on the invoice number that you have received payment for.
Under the Payment history table click on the Options link.
A new set of options should now appear beside the Payment history table. Click on the FX button and enter a date and description.
Click FX Write Off to complete.