In carrying out the provisions of subchapter II of this chapter with respect to insuring mortgages secured by a one- to four-family residence, the Secretary may not exclude from the principal amount which may be insured under such subchapter any sum solely on the basis that such sum is to be paid by the purchaser to a broker who has been the purchaser's agent in the purchase of the residence, but the principal amount of the mortgage, when such sum is added, shall not exceed the limitation as to maximum mortgage amount provided in subchapter II of this chapter