Is it Islamically permissible for one of the heirs of a deceased father to retract the sale of an apartment to his brother, who has paid only a quarter of its price, and to demand the return of the apartment in exchange for the amount paid, taking into account that the sale was at an old price and that the father was financially lenient with the purchasing son?

If the sale between your father and your brother was completed, you are not permitted to revoke it except with your brother's consent, and he must pay the remaining price. The sale thus becomes binding on both parties. The existing inheritance must be divided among the heirs according to Islamic law. Your sister's share of the apartments is one-fifth of these apartments if the heirs are limited to you, your brother, and your sister. It is also permissible to agree on dividing the inheritance by giving your sister the value of her share of the apartments.