If you keep it, it's theft. There, I said it. You hold a legal trump card but it doesn't change the moral implications of the deal.
They made a mistake and he's doing them a solid by sending it back. Words have meanings and these days it's important we recognize those meanings. They legally sold him a part for far less than they paid for it. Theft is the act of stealing. That didn't happen here. People can argue about the moral implications but in doing so we shouldn't turn it into something it's not.