If you have redeemed the wrong gift certificate by mistake and if it has not yet been invoiced, you can reverse the redemption on the same day. Highlight the gift certificate in the gift certificates-dialog or in the list box in the guest profile and choose the option cancel redemption from the context menu. The contra-posting of the redemption on the respective account is deleted and the gift certificate is valid again.
To correct a faulty redemption after the night run, you have to settle the gift certificate service on the account (using payment type cash, if in doubt) and issue a new gift certificate (using the same payment type).