The currency is wrong for this sale
Sales transactions must use the currency assigned to the customer in the Profile view of the Card Information window; you can change the exchange rate used for a particular sales transaction, but you cannot change the currency being used.
If this is a new customer and you've never recorded any transactions for them, you can display their card (by clicking the arrow that appears before their name at the top of the Sales window) and change the currency there. Once you've begun recording transactions for the customer, however, the currency cannot be changed. If you do business with the customer in multiple currencies, you may need to create a separate customer card for each currency you use.