Ever get cheated on a restaurant gift card?
We got a $100 gift card to a restaurant chain and used $40 one night for food and left a cash tip. They give you a credit card looking receipt and we just left it without doing anything. We didn't understand how it worked. Well, we thought the amount available was on the strip on the card but apparently it's in the restaurant's system. Anyway, when we went to use the card just 2 weeks later, there was only $40 left on it. Someone who worked at the restaurant nailed us for another $20 in addition to the cash tip we left and of course given that we left the paperwork there they obviously knew we would have no way to prove it. Lesson learned, take your receipt when you pay with a gift card card because the restaurant staff will skin you whenever possible!
Restaurant gift cards I've used only allow you to purchase food or drink and they can't be used for tips. So it's unlikely that an employee benefited from an additional $20 tip. Two things could have happened; One- you thought you spent $40 and they billed you $60 and it was a mistake or two- leaving the receipt behind an unscrupulous employee used the account number on that gift card to charge $20 worth of food for a friend.
You should go back and have them check their records. Should be no problem for them to give you a duplicate receipt or transaction dates with purchases.