242.26—Agreed interest rate.
(a)
The mortgage shall bear interest at the rate or rates agreed upon by the mortgagee and the mortgagor.
(b)
The amount of any increase approved by HUD in the mortgage amount between initial and final endorsement in excess of the amount that HUD had committed to insure at initial endorsement shall bear interest at the rate agreed upon by the mortgagee and the mortgagor.