What should I do after using a counterfeit banknote to purchase an item, and receiving a refund from the seller, knowing that I have already given in charity an amount equal to my share of the refunded money, but I do not clearly remember the seller?

Dealing in counterfeit currency is unlawful (haram) because it corrupts money and harms people, as the Prophet, peace and blessings be upon him, said: "Whoever cheats us is not of us." There is no sin upon a minor (non-baligh), but he is obligated to return the money to its owner, and he must guarantee the full amount. He may then reclaim from his partners what he paid on their behalf, as each one of the thieves is liable for all the money they stole. Charity is not sufficient to fulfill the obligation of returning what is due to its owner if that is possible.