(a) The mortgage shall bear interest at the rate agreed upon by the mortgagee and the mortgagor.

(b) Interest shall be payable in monthly installments on the principal amount of the mortgage outstanding on the due date of each installment.

(c) The amount of any increase approved by the Commissioner in the mortgage amount between initial and final endorsement in excess of the amount that the Commissioner had committed to insure at initial endorsement shall bear interest at the rate agreed upon by the mortgagee and the mortgagor.