Step 1: Add the foreign currency.

1. Follow How can I add a foreign currency?

Step 2: Setup customer or vendor to use foreign currency.

1. Open the customer or vendor record.
2. Select the Options tab.
3. Change Currency to the foreign currency you want to invoice with.

Notes:

1. Changes to Currency can only be made if the customer or vendor has no transactions entered.
2. If you can't change the currency, see How to change vendors or customer's default currency.