Is it permissible for the heirs to annul a contract for the purchase of land, given that the deceased passed away before taking possession of it, knowing that he had paid installments of its price, and it was not ready at the time of his death?

The land purchased by the father before his death is considered part of the estate, and the right to it transfers to the heirs, to be divided among them according to their shares. The heirs do not have the right to return the land unless the seller agrees to annul the sale. The father's not taking possession of the land before his death does not affect the validity and binding nature of the sale, even if the purchase was for an undivided share.