Are the heirs obliged to complete the land purchase contract that their deceased father entered into, or do they have the right to annul it?

The heirs step into the shoes of the deceased in receiving the land after paying the remainder of its price, because it is mortgaged for it. Mortgaging a sold item for its price is permissible, and the mortgage is not invalidated by the death of the mortgagor. The heirs do not have the right to annul the contract except with the seller's consent.