Is it permissible to buy a gas station mortgaged to a bank in installments, while paying usurious payments?

It is permissible to purchase the station by installments as long as the contract is valid and free from forbidden conditions. The fact that the seller owes usurious debts to the bank does not prevent the validity of the contract. However, if the station is mortgaged to the bank, it is not permissible to purchase it except with the bank's permission, because the mortgagor is not allowed to dispose of the mortgaged property in a way that removes ownership without the permission of the mortgagee.