I am saddened by the responses....First--WHY does he not want to go back? You have never mentioned this.. what happened? It could be something that he has every right not to want to be around.
Second--you are his parents, and in this you decided to spend the money--this is not something that a child "pays back" are you going to make him pay you back for the clothes and food you have bought him the past 12 years, as well? This is one of the responsibilities YOU take on as a parent.
He is also only 12--they do change their minds, and yes, need to finish what they start--but WHY doesn't he want to finish it? Have you found this out? Have you actually opened your ears to him and really listened to why he does not want to finish this (or whatever else you feel he does not want to finish?) This will also show him that you care, and that it's not all about the money (in which he will learn that money is more important than feelings)
Ask yourself is the battle worth it? Seriously? To have you AND him angry over something that should be fun and enjoyable?
I would start by saying you are sorry he had such a bad time--and that you are sorry for getting angry Then ASK him what happened, and LISTEN. If the reason is legit, swallow the fact you spent the money and enjoy the time you have with him. This time passes SO fast and soon he will be off on his own...or something may happen that makes you regret arguing over something so trivial...
It sounds to me like you care more about the fact you spend $100 on this than how this whole thing makes him feel.
Ask--Is this really the battle I want to have with him? And remember he is only 12--he is not an adult, so don't treat him like one.