Hi there, @Cityconect,
Fiat Lux - ASIA is right. You must set up the Multicurrency feature to assign a different currency to a customer. Here' are the steps to do that:
- Go the Gear icon and select Accounts and Settings.
- Tap the Advanced section.
- Click the Currency window, and mark the Multicurrency box.
- Agree with the terms and select Save then Done.

To add foreign-currency Vendors or Customers:
- Go to the Sales menu and select Customers.
- Select New Customer.
- Enter all the customer details and go to the Payment and billing section.
- Add the currency in the This customer pays me with field.

- Select Save.
Please note that we can only assign one currency per customer. Once it is set, you're unable to change it with another one.
If you have any questions about this, feel free to get back to this forum. I'll be right here to help you any time.