(a) The interest reduction payment to the mortgagee shall be in an amount not exceeding the difference between the following:

(1) The monthly installment for principal, interest, and mortgage insurance premium which the mortgagor is obligated to pay under the mortgage; and

(2) The monthly payment for principal and interest the mortgagor would be obligated to pay if the mortgage were to bear interest at the rate of 1 percent per annum.

(b) Where individual family units in the project are sold, subject to a plan approved by the Commissioner, and as the principal amount of the mortgage is reduced by payment of the portion of the mortgage attributable to the sold units and as the amount of the mortgage payments which the mortgagor is obligated to pay is reduced, proportionate reductions will be made in the interest reduction payments.

(c) In addition to the interest reduction payment referred to in paragraph (a) of this section, the mortgagee shall be entitled to the monthly payment of an amount the Commissioner deems sufficient to reimburse the mortgagee for its expenses in servicing the mortgage.