Is it permissible to buy an apartment in installments from someone who bought it with a mortgage, knowing that he has not finished paying its installments and will pay them from the installments I pay him, and on condition of not dealing with the bank at all?

It is permissible to buy an apartment that its owner purchased through real estate financing, whether the financing was a legitimate Murabaha or an usurious loan. This is because the prohibition is related to the buyer's liability, making his ownership valid, and the sin of usury falls upon him. Ibn Habib and Ibn Rushd mentioned that the forbidden accrues to the liability of the seller and the giver, and they are responsible for it. It was reported from Ibn Mas'ud that he said: "For you is the enjoyment, and upon others is the sin."