What is the ruling on someone who received a down payment for the sale of a property, then decided to buy it for himself and returned the down payment to the buyer, who is displeased, knowing that it is customary for the sale contract to be concluded upon receipt of the full amount? And what is the expiation for this action?

A broker is a trusted agent, and it is forbidden for him to betray or be disloyal. Your purchasing for yourself what you were commissioned to buy is unlawful according to Islamic law. Some scholars have permitted an agent to buy for himself what he was specifically commissioned to purchase. However, the situation described is unlawful due to the treachery involved and buying over your brother's purchase after the price has been agreed upon and a down payment made.

If the unlawfulness of the questioner's action is established, then his expiation is to repent to Allah and resolve not to repeat such an act. If the sale took place, it belongs to the principal according to the Hanafī school, or it is void according to the Hanbalī school because it is a sale over a sale during the option period.