I'm home. It went pretty well, things started off rough with lots of arguing on his part, but I kept my cool (except for the fire red cheeks I'm sure I had) and stood my ground and told the truth, and when all was said and done, he realized that he really did owe me money.
The mediator said that I could go forward with the drug testing, but since he has supervised visitation, and since I can refuse him visitation if I think he's high, there really wasn't any point. But, interestingly enough, he refused to do it anyway, and the mediator said "your refusal tells me you are still using drugs." I had to laugh (not out loud!) at that. Of course it does.
So in the end, he agreed to pay me back that money within 6 months, and I showed him that I will stand up for myself. We had a nice talk afterwards about getting along and communicating, and I think this was very good. I'm also so, so glad it's over.
Thank you for all of your support, it really means a lot to me.