Purchase transactions must use the currency assigned to the vendor in the Profile view of the Card Information window; you can change the exchange rate used for a particular purchase transaction, but you cannot change the currency being used.
If this is a new vendor 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 Purchases window) and change the currency there. Once you've begun recording transactions for the vendor, however, the currency cannot be changed. If you do business with the vendor in multiple currencies, you may need to create a separate vendor card for each currency you use.
This tip applies to:
Purchases window -- all layouts