If a card has the wrong balance and a manager has signed off on adjusting it, here is how you do so. Not everyone has access to this module.
Go to Accounting -> Receivables -> Adjustments
Check the balance by searching the GC number in the client lookup
Go to Adjustments in the same module
Add a new adjustment for the Card you have. Adjustments when adding funds to the card has to a negative number, removing is a positive number
The gift card should have the proper amount on it now.