You can assign a default currency for your customers for ease of use in creating Sales orders. This is especially useful when selling to international customers and making sure your default sales price is always converted by the most recent currency rate.
Adding a default currency for customers
Navigate to Contacts screen > Customers table.
Choose the currency in the Currency field of a customer, and this will be the default for that customer.
When you have added a foreign currency to your customers, you can go ahead and start selling in multiple currencies.