long answer
I have found many times the “it’s against the law here” claim is simply a lie. All the chain store that are known for lying seem to convieniently have laws in effect on their store that do not apply to other local stores. Because other local store are truly based on customer satisfaction rather that use advertisements to make it seem that they are. Usually the its against the law claim is a cop out. Your best bet is to complain loudly and forcefully. If you are a big enough pain or make enough of a scene in the store (not so much as to merit them to call the police) odds will usually fall your way.

As an alternative i suggest selling it for a small loss. Say its a 25 dollar gift card try to sell it on craigslist or the pennysaver if it is published in your area for $20.