@Amber603 Before I go into "here's where you went wrong" mode, this is the good news: that informal offer you made to throw money back at the guest if you rebooked is not binding, so you are free to block further contact with this person and keep the payout that is rightfully yours.
And now, the 5 words you don't want to hear: here's where you went wrong. The guest had already cancelled, so there was no reason under the sun for you to offer him a cut of your earnings for whatever booking you could scrape together in his wake. That whole maneuver of offering a refund if dates are rebooked is just a tool to persuade people to process a cancellation and get your calendar open. It's not something you're expected to do when the cancellation is already done.
Absolutely don't let this person book again; you're guaranteed nothing but trouble. As long as he canceled before his original arrival date, he can't review you, so that's not going to be an issue as long as you decline, block, and forget about him.