AITA for refusing to pay back my friend

For context this has happened a few months ago but it's still brought up so i just want to know if im in the right or not

I've never countet exactly how much my friends owe me. It was always a give and take, i'll pay this time they pay the next time. This system has always worked because my friends have always repaid the favor.

This one friend and i have been on a training course for six weeks and ordered a lot of takeout in that time We swiched who payed every day so that we are even in the end.

Now at the end of the course he comes up to me and says "you owe me 165$". Now im conflicted because i didn't keep my receipts and therefore can't prove to him that i payed the same amount (if not more) for his food.

I tried talking to him but he just says "If you can't show me how much i owe you, then thats on you"

AITA if i don't pay him back?