You can open a Client Card in Billings Pro and assign a currency to the Client. The list of currencies displayed matches the ones you've set up in the Preferences. All Estimates, Statements, Invoices, Payments and Retainers for that Client are handled exclusively with that currency.
Note that new Clients are always assigned the base currency.
To assign a currency to a Client
1. Open the Client Card.
2. From the currency pop-up menu, select a currency.
3. Click OK.
From the currency pop-up menu, you can select "Add Currency" to set up a new currency. This opens the currencies preferences pane.