As always in this business, facts & circumstances. Why did you fire him? Is that still the case? Why isn't the new preparer doing the work? If you do this, and he decides he likes you better, is he welcome back for good (if not, tell him up front)?
Up to you, but definitely get paid - and make sure the check clears - before you do anything. And don't let him pay via PayPal, as he can go back to them after you're done and claim he didn't get what he paid for. They almost always decide for the payer, and you're out the money plus a Pp fee. Happened to a colleague of mine; I took the Pp link down from my site the next day.