But the fact of the matter still remains, you are being paid by the manager of the restaurant. If he see's you having the spectators doing tricks, he may rethink your agreement. I doubt he would be willing to pay you to watch people do magic tricks.
First, a manager won't see a spectator doing a trick and you watching. For all he/she knows, you told the spectator to take the cards and you were just giving them instructions on what to do. There are a lot of tricks just like that.
As for being paid to do magic, someone said it...you're being paid to entertain. Letting a cocky spectator see the difference between a card trick and magic entertainment is not inappropriate. It's a rare situation, but I will let them. People always suspect "special magic cards". How many times has someone asked you that. If someone ask if they can show me a trick, I give them the satisfaction and then I awe the crap out of them with some real magic entertainment. I don't see anything wrong with it. But like I said, the request is very rare.