If the leader of the event didn't know whether it was going to be paid, he should have told the OP up front. If the guy was going to play as a volunteer, he should have been told up front. Regardless of your religious background, to tell one person one thing and then do another is dishonest, no matter what you wrap it up in. I'm sure that pay was part of the decision. If he just wanted to worship, he could do it at his own church or according to his calling, not at the calling of someone who said they were hiring him.
It's one thing to show up at church to worship. It's another thing to attend regular rehearsals, wake up at the crack of dawn to drive to church, and being there for there for most of the day.
+1
I personally find the idea that churches can't (or shouldn't) pay their musicians quite laughable. In my home town, it's not uncommon to drive down a mile stretch of road and pass five to six multi million dollar churches. It's clear that these churches are ran like businesses and that they use musicians as a means to help their business prosper and gain new 'customers'. If some musicians want to volunteer their services for their church then that's fine, but if they say they'll pay someone for their services then they need to pay them and not try to hide behind their scripture as reasons why they shouldn't pay the other party.