He's not going to get in trouble for it. It was the store's mistake, not his. "The fuzz" isn't going to get anyone. What a stupid thing to bring up.
And the right thing to do would be to keep it for yourself. All these companies care about is making a profit, and they'll do it at your expense if possible. Think of the phrase "turnabout is fair play". They deserve it if they had an employee incompetent enough to screw up the sale like that.