I think the problem is this: the man was paid for his work. People don't seem to get that.
The deal was that he was paid an amount of money to make an art piece. That art piece was supposed to use another bunch of money as props. He was supposed to then give back the prop money after the exhibition was over.
When he made his work that used none of the money, that was fine. The museum rolled with it and gave him his dues. They didn't even ask for the prop money back when they realised he wasn't using it.
The problem is that he's now supposed to return the prop money that was to be used in the artwork, and he's refusing to.
He's already been paid, he's just being a shit to an organisation offering a public service.