In the event that the Commission finds that financing on reasonable terms is not available from other sources, the Commission may, in order to facilitate the sale of residential property under subchapter IV of this chapter, accept, in partial payment of the purchase price of any such property notes secured by first mortgages on such terms and conditions as the Commission shall deem appropriate. In the case of houses and apartment buildings, the maturity and percentage of appraised value in connection with such notes and mortgages shall not exceed those prescribed under section 1715n(a) of title 12, and the interest rate shall equal the interest rate plus the premium being charged (and any periodic service charge being authorized by the Secretary of Housing and Urban Development for properties of similar character) under section 1715n(a) of title 12, at the effective date of such notes and mortgages.