Customer Card
On the customer card, under the Finance tab, you select the currency in which the customer will be invoiced. This can be overridden at the order level. When changing the currency, all existing orders must be updated manually. All new orders will retrieve the currency from the customer card.
Order View
Under the finance tab in the order view, the currency is retrieved from the customer card, but you can override it to another currency per order. You can also choose to enter the exchange rate manually by unchecking "exchange rate from register". This setting cancels the inheritance from the register. The exchange rate remains unchanged and must be updated manually by the user.
In the order view, above the price columns, you also see which currency the order has. Prices are displayed in the selected currency, but you can uncheck "show prices in currency" to see the prices in your base currency.