Is it permissible to mortgage land to the bank until its price is paid in installments, and is this considered a contract within two contracts if the amount is paid early and the mortgage released?

It is permissible for the seller to stipulate that the sold item be mortgaged with him until the price is paid in full, which is what the Islamic Fiqh Academy has adopted. There is no harm in expediting the payment to release the mortgage, or releasing the mortgage before payment with the bank's consent.