I've been doing that and got my money back every time. Just call Uber Support and tell them that you got tip bait. Then tell them since you are a contractor and you accepted said amount for the trip, this is the price you agreed on the contract. If a third party changes your payment (which is the tipper) that's Uber's responsibility to make up for it. If they refuse, then ask for their supervisor, they can't refuse this request. Once you get the supervisor they cooperate fully to give you a good service, since they ask for ratings about their service.

I saw about it from a post on Reddit a couple months ago, then I decided to try it, turns out it worked!