A Law question, please help?

Adam is the religious leader of the religion that Bob and Carol practice. They want to get married so they ask him to perform the ceremony. He agrees after describing them that he takes such duties very seriously and spends a lot of time and go writing a sermon, thinking about the couple's particular personalities and preparing some materials he like to give to the couple. He also tells them that the charge for his service is $ 2,000 and they agree. The morning of the ceremony, Bob and Carol have a huge argument and dissolve their wedding. Adam demands his $ 2,000 for everything he has prepared but they refuse to payment, saying that they agreed to pay him the money for marrying them, which he be not going to do. How could Adam have avoided this mess?
By writing up a contract that stated the couple had to pay 1/2 the cost up front Who care..he's a religious nut. He should've married them for free if he practiced what he preached
Gotten the money upfront? Or a downpayment..A contract? If it's not surrounded by writing it never happened.
Sheesh. There's a lot of ways Adam could have avoided this mess.

One is by instructing the two of them not to have any contact beside each other the morning of the ceremony, as most traditions dictate.

What a charlatan.
Adam? Okay, I'll bite. I guess that was the wrong thing to say within that situation.

The minister can afford the write-off at that price.
First off, a written agreement is critical for such things. In said agreement, Adam would have be wise to include that the fee included not just the exploit of marrying the two, but the research and efforts that he would incur in preparation thereof. Also surrounded by the agreement should be a clause regarding how much Adam should be paid if the couple should decide to not attain married (for whatever reason) after he has performed the preparations. Finally, a deposit against the charge would have been appropriate for Adam's protection in luggage just such a problem were to occur.
A written contract, or tort is part of the answer. However, he also needed a clause in said contract specifying damages or penalty for default. Then he could sue for breach.