The system allows to invoice boxes, units and hard goods in your customers' currency. This option currently works for Euro, United States Dollar, Chinese Yuan, and South African Rand. Before this option works, make sure of set up an exchange rate and add the appropiate foreign currencies. Read Exchange Rate and Multicurrency Setup for further information.
If users create and invoice with an exchange rate and then perform any change on it with a different exchange rate, this could affect the totals since several products within an invoice would have different exchange rates.