Seriously, you can stick the IOUs in a conspicuous place in the classroom or the company. As long as he feels a little ashamed, you can get the money back (you have to open another piece of paper to explain that he is paying back the money, otherwise people will think you are crazy). If he is really thick-skinned, you have to take extreme measures, such as knocking on his door every day and calling him in the middle of the night ... Besides, I don't think you can go to a debt collection company no matter how much money you owe. I think that's too dangerous. As long as violence is not used, the method quota should be completely fine. You are evil, I am more evil!