Dude, you should've done what your mom proceeded to do, have the sense to not leave the issue right then and there. I would've demanded to know when he was going to pay you back. Now he won't take you very seriously unless you do something drastic, like calling the cops.

I'd go back and demand it, and if all else fails, get the school involved, since it happened while you were in school. You can't be flacid about it, demand it.