Is $900 really life changing money here? Sounds like the guy was just trying to flip your car for a quick profit, and the offer probably wasn't going anywhere serious. Finish the build if you've already dropped money into it, because bailing now just hands him your work for cheap.

Does the money really matter more than what your brother got out of it? Honestly I don't think paying a canceled person is some big moral failing, especially when the guy was already getting trashed everywhere and probably needed the cash more than most. You didn't fund his old tweets, you bought your brother a gift he loved, and that clip sounds like it hit the mark. People act like a dollar spent is a vote you cast forever, but sometimes it's just a transaction between two humans and the good part outweighs the messy part.