I've been jerked around by those sorts of situations, as a musician, a few times. But - these things happen. "How binding" the agreement is legally, I have no idea - I suspect not very - but there is an ethical onus on the club to somehow "make it right". I would suggest that that would be by, first of all, apologizing profusely, and then offering the performer a different date, with as sweet a deal as possible - or as is reasonable. Even if the committee isn't enamored with this performer.

It seems to me that, under the circumstances, and with the circumstances explained to the performer, two months is plenty of notice. Accept that he's annoyed, and has perhaps said or will say something intemperate, and try to repair the damage. Remember: a soft answer turneth away wrath.