Someone paid for my drink, what should I do?

Ok so brief this guy from college I won't call him a friend, he's my classmate and we were just hanging all together as a group before a class and he was going to the canteen and he asked all of us if we needed something I denied 2-3 times at the end he said I'll just get you a diet coke. Cool but now we got consumed with something and now I'm back home all I can think of is i didn't pay him back or even asked. I could it next time I meet him right? No i can't because this was end year and it's like a dummy college so we won't meet again as I said we aren't friends. Now in overthinking should I text him for his QR ( we have never texted or shared numbers I'll simply find it in group chat ). Ik the amount is small but it's kinda eating me up that someone whom I'm not close to paid for me. Should I text him ? It's gonna be awkward i think specially because he's like a rich dude and probably in their friend circle it's common. I mean it's common in my friend circle too but we are close. And I am not with him. What to do ? Yes I think aot