be diplomatic

It depends on the size of the project.  If it takes less or about your remaining time in the old team, tell him to have another one to shadow you. And tell him it is good for his team for later support. But in reality, you do not have to finish and you will have the other guy to pick up what ever you left. If you feel he is trying to retaliate you by an unreasonableamount of work, let your new boss know first so your old boss will not have much credibility if he tries to bad mouth you in your review time.

Talking about review, you would be careful. If your company has bonus and part of it comes from your old team according to the proportion of time spend on each team, you may just suck up and finish it and let the guy know you are doing for him (instead for your year-end bonus).