With refunds you have two options. To use the giftvoucher app to give a full refund, or to use stripe to give a partial refund.
A full refund can be performed by finding the voucher and clicking refund. This will trigger a full refund from Stripe and will be reflected in your voucher account.
You can find the transaction in Stripe and give a partial refund on any transaction. Please note that this won't be captured in the gift voucher app.