How to default the Invoice currency

We are on 12.1.3. We use Receivables but not Order Management. We currently bill our customers only in USD. We currently have a single Ledger with USD as the functional currency, and a single Operating Unit.

We are looking into ways of using the multi-currency functionality in Oracle, using the single ledger and operating unit.

We have international customers whom we plan to bill in the foreign currency.

I went through the different customer screens and did not see a field for the default currency. How to we automatically default a foreign currency on a customer's invoice? In AP we can set a default for the invoice currency on a supplier site. How do we do the same for AR customers?