Are the brothers entitled to object to their father selling his apartment to one of his sons at a price lower than its current value, and must the son compensate his brothers?

If the father sells his apartment to his son in a valid sale, then the apartment becomes the property of the son and does not fall within the father's estate that is to be divided among the heirs. However, if the sale was a sham sale or a preferential sale, each has its own specific rulings. It is advisable to refer to the Sharia courts to settle disputed cases, because the Mufti hears from only one party.