I've had more than one clerk say that they were required to keep the gift cards even though the balance was ZERO....from one property to the next, there's very little consistency. The good news is that you can clearly tell if the clerk did the right thing by getting a new receipt after the gift card transaction and matching up the last four digits on the folio (under VISA which is how gift card redemptions appear) to the actual card itself. Doing that, even when paying the reminder with a "real" visa, has helped determine if the clerk made an error or not. It's also proof for if there are any issues later.