In the "Add Gift card transaction" one can select multiple currencies, but if another currency then the one on the gift card is selected, an error is presented. To mitigate the error I suggest setting the currency as default value. Also the field could be locked for the user, since he can not really change anything?
Presenting the information what currency the transaction will be in is crucial though.
I assume the field was not locked, since another Giftcard with potentially another currency can be selected. I don't see that workflow to ever really be used by users. Rather a user goes back to the overview and renters the form again with the preselected gift card. Therefore we also could lock the gift card field?