ANSWER:

You must contact him and return the money, no matter how insignificant the amount is.

However, if he says no need to return then you are free from that debt.

QUESTION: I remembered that I owe a non-Muslim boy who used to go to class with me one euro from around 2018 if I remember correctly. If I were to contact him again I already know he would not accept because it is very little money and he doesn’t even remember lending it to me. What should I do?