From this week's Times Ethicist Section, here is the kasha about a 'Rabbi who backs out last minute':

My son and daughter-in-law paid a rabbi in advance to officiate at their wedding. A few days before the ceremony, the rabbi was incapacitated and could not attend. She reluctantly agreed to return part of the money, arguing that she was entitled to be paid for her preparation time. I say that she did not do the job and so should not collect a fee. Right?