Mismatching Currencies

At this time it is not possible to have a credit invoice using one currency applied to a subscription invoice using a different currency. This is due to Recurly not internally converting the currencies to equal the exchange. In any case, the currencies on both the credit invoice and the subscription invoice must match in order for the credit to be applied. 

This is the same behavior for gift cards as well. If you were to try to apply a gift card with a USD currency to a GBP invoice the transaction would not be able to take place. The currencies would have to match here as well.