Remove/Adjust Gift Card credit Balance from a customer

Hello,

I have a customer who received gift vouchers for my shop. She redeemed those not online, but IRL in my shop.
I now need to adjust the funds on her account accordingly. I really have spent 2 hours searching and reading this forum, but I must be overlooking something. Deleting her account seems not the right way to do this .. :)

Any help would be highly appreciated, version (TEXT_CURRENT_VER_IS v1.3.9f) ( yes i know, old version, needs updating.. if i ever have a bucket of time..)
Kind Regards, Niek

There's this. written for an earlier version of Zencart:

https://www.zen-cart.com/downloads.php?do=file&id=373

I strongly urge you to create a test installation of Zencart to test this mod on, if you choose to try it, and not install it to your live site until you're sure it works properly.

I haven't used it.