Buying and Selling: Returning the Commodity or Money Taken from a Non-Bāligh Child
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2093
If a person buys something from or sells something to a non-bāligh child when transactions with such a child are not valid, he must return the commodity or the money that was taken from the child – in the event that it was the property of the child – to his guardian. If, however, it belonged to someone else, he must return it to its owner or obtain the owner’s consent. In the event that he does not know who the owner is and does not possess any means of identifying him, he must give the thing he acquired from the child to the poor on behalf of the owner as radd al‑maẓālim.[1] And the obligatory precaution is that to do this, he must obtain permission from a fully qualified jurist (al‑ḥākim al‑sharʿī).
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling
Radd al‑maẓālim refers to giving back property – which has been unrightfully or unknowingly taken – to its rightful owner, or if that is not possible, to the poor as ṣadaqah on behalf of the rightful owner.