You may want to look up the "Consumer scanner law" or something like that. There is a law on the books where if scanners are not programmed correctly and ring up the wrong price you are entitled to something like double or triple the difference back. Then you can say to the manager, "it's not just 2 bucks anymore is it?"

Plus, if it WAS only two bucks, what the hell was her problem in giving it back to you?!