You can sell and purchase in multiple currencies. One currency must be designated as the company’s local currency. Tax, sales and purchase reports use the local currency.
If an invoice or bill is issued in a currency other than the local currency, the current exchange rate is captured and stored in the document. When the payment is received later, the exchange rate usually differs. When the payment is allocated to the invoice, the resulting foreign exchange gain or loss is recorded in the allocation entry and can be used in reports.
Documentation
See more about multi-currency in the documentation.