I bet they would still take it back, even if not for credit. After all, that is a fish they would be able to sell twice...and they like money.
Or, if they won't, try another store. I'm positive that someone, somewhere will take it!
You don't want your angelfish to get all torn up by it! But, if, for whatever reason, you can't find a place to take it back, its a good reason to get another tank!!!