What is the ruling on a received gift that was given by a brother for the purpose of assisting with marriage, but the assistance was not provided; and must it be returned to the giver who cannot be reached in order to clear oneself of liability?

If a gift was given to someone to perform a task, and they did not perform it, then they must return the gift if it is still extant. If it is damaged, they must return something similar to it or its value. If the owner of the gift cannot be found, it should be handed over to their heirs. Otherwise, it should be given as charity on their behalf.