What is the ruling on selling an apartment in installments before taking possession of it, where the first buyer paid a portion of the installments, then sold it to another person at a higher price for them to complete the remaining installments, and is the profit resulting from this sale lawful?

Whoever buys something, and it enters into their possession, is permitted to dispose of it by selling or otherwise. The incomplete payment of the price does not prevent the transfer of ownership of the sold item to the buyer, unless the apartment is mortgaged for its price until its full payment is completed. In that case, it is not permissible to sell it before the mortgage is released, except with the permission of the mortgagor. It is permissible for the seller to stipulate to the buyer that the sold item be mortgaged with them to guarantee their right to collect the deferred installments. The sale of a mortgaged item is valid but contingent upon the approval of the mortgagee.