This option, including a choice between singular currencies, is available in the section Basin Settings right during issuing of a new invoice. This section is located in the upper first box.
In the offer are many currencies, and which exchange rate is calculated by the current exchange rate.
Everything is updated in case of filling in singular items of invoice.
This amount in other currency will invoice be saved in the list of issued invoices. Amounts are not recalculated here (The amount stays the currency, in which the invoice was issued in).