How to solve this art commission issue?

I’m a graphic designer, and sometimes I do private commissions for people on the side. For logos and other designs that a client plans to reprint, I charge upwards of $250—$300. For small things that are just for the client, the price usually hovers around just $100.

Here is my problem.

I was contacted by someone I know for a private commission. At no point when we were discussing the project did he mention anything about reprinting or selling. He is trying to make an independent movie and he said he just wanted to have some artwork of his characters. So I charged him the smaller fee. He paid me $50 and is to give me $50 when I’m done. I sent him a progress update about a week ago and found out yesterday that he posted it on his facebook. He tagged me in it and then stated something in the post about how he plans to sell them as posters to raise money for producing the film. However, this was never mentioned to me until now and I don’t know what to do…..

Normally I make just my bigger clients sign an agreement, and this is the first issue I’ve had with a smaller private commission being replicated without my permission. I’m not sure how to handle it. Especially since a lot of people are really liking just the progress picture alone and are already promising money for it when it isn’t even his artwork to sell as a print. I make my pictures for my client’s private use only, not to sell copies. Does anyone have any suggestions?? Or is there nothing I can do but continue with our initial agreement?