I'm going to make my girlfriend return an expensive gift she got.
Why do you want her to return an expensive gift?
It's a waste of money, I can buy something more useful with that money.
I can see why you would feel strange about keeping something expensive, but it's rude to ask her to return it. It might make her feel as though you don't like what she picked out for you. Just keep the gift as a treat.
I'm going to go to the manager and try to solve this.
While it might have cost a lot it's not up to you if it's returned or not. It was a gift for her and she should be able to keep it.