If there's any difference in the exchange rate when the payment is received, the difference will be added to Chart of Account accordingly as Realized Currency Gain/ Loss. You can see the details in Journal Report.
For instance, an invoice was created in USD for Atop Security. The exchange rate was 1.34 when the invoice was created. But when the payment was made, it was 1.54. In Journal Report, the difference is added to Realized Currency Gain/ Loss as Credit.