Honestly, this shouldn't even be a question. If he owes you money, just ask him. If he has problems returning the money because he doesn't have it, he won't have a problem explaining it, since we can all face a similar situation. But if he doesn't want to give you answers and has no reason not to return the money, you should ask him until he returns it to you. After this, you decide whether to continue being friends with someone like that or not. In my case, I clearly wouldn't be friends with someone like that.