My Classmate Owes Me $50 and Refuses to Pay Up!

So I had this really annoying classmate named Alex who's always borrowing stuff from everyone without ever returning it or paying for what they borrow. It drives me nuts because the guy acts like he owns everything and everyone, but he's not very good at sharing responsibilities when it comes to money. One day last month, we were out shopping and I bought a bunch of snacks for our study group. Alex had his wallet with him, and he specifically said that since he was going to be late because of work issues, he'd give me $50 as an apology and cover the costs for everyone on the next outing. We had another meeting this past week, and Alex still hasn't paid up. He even laughed when I brought it up in front of the group! Can you believe this? It's like the guy thinks my money means nothing because he doesn't value his own contributions to our friendship or study sessions. Here I am, wanting to just enjoy some fun snacks and hang out with friends without worrying about getting paid back. But Alex keeps hitting me up for favors while acting all high-and-mighty when it's his turn to contribute something meaningful like money. Am I the asshole here for being persistent? Is there a better way to approach him without coming off as too aggressive or desperate? Your thoughts, please share.