A while back I laid out money to get a gift for someone from a bunch of us. All paid but one, who remembered last week to pay his portion. He had the email how much it was (it was dollars and cents) and he sent me a Venmo for the amount rounded up (for convenience sake, not because he wanted to pay me more for the time delay in re-paying). It is a difference of less than 50 cents on $33.
Essentially when you laid out the money you gave your friend a loan, and now he is paying you more than what he got, which is ribbis. Although with items, i.e. it is permitted to return an egg that is slightly larger, this is because it is hard to get it exact, and the person is mochel. With money though, it is evident that he is paying you back extra, therefore when returning money it has to be exact. Even if your friend is mochel the money, you may not take ribbis. You can call your friend and tell him that the extra money will be given to tzedakah on his behalf.
Kesiva vchasima tova
Bris Pinchos 5-17,18