Must a sum of money taken from a person years ago be returned, given that the thief was not an adult at the time, and knowing that the amount is very small and the rightful owner can no longer be found, and what is the punishment for the oppressor on the Day of Judgment?

The Pen is lifted from the child until he reaches puberty, so there is no sin upon him if he commits a forbidden act before reaching puberty. However, guaranteeing the rights of people and what he damages of their property or usurps is incumbent upon him. He must return what he has usurped, and if he is unable to return it, he should give charity on its behalf.