Is taking money for a reservation not paid for by the restaurant, which was made as a courtesy, considered unlawful?

You must return the money to its owner, as you are his agent. If the manager is not authorized to dispose of food without charge, then you are not permitted to withhold payment from the restaurant. Your liability is cleared by paying the restaurant, not by returning the money to your client. However, if the manager is authorized to do so, there is no objection to accepting his offer, but you are obligated to return the amount to its owner.