You can tell them that, they probably need a laugh. I seriously doubt that whatever "mistake" they made is going to result in you running off with THEIR money without recourse. Your post doesn't have enough details to make a valid decision. It could be that whatever the defect is could allow them to foreclose on you if you don't comply. If you can convince them to make it worth your while (same or better terms, having them cover all the costs, etc...) why wouldn't you do it?