What is the ruling on selling an apartment to one of the heirs with deferred payment, then selling it to another heir without the knowledge of the first, and what is the ruling on the first buyer retracting his approval of the second sale after being forced into it?

If the apartment was sold to one of the heirs, it becomes his property upon the transfer of ownership by contract, even if the price is deferred. It is not permissible for the other brother among the heirs to sell the apartment because it has exited their ownership.

If the first buyer agrees to the heirs selling the apartment to the other brother, this implies the annulment of the first sale, and this annulment is valid. The sale of the apartment to the other brother is also valid, and ownership is transferred to him. The first buyer then has no right to retract, unless the second sale is annulled and he re-purchases the apartment from the heirs.