The Secretary shall require any purchaser of a multifamily housing project owned by the Secretary which is sold on or after October 1, 1978, to agree not to refuse unreasonably to lease a vacant dwelling unit in the project which rents for an amount not greater than the fair market rent for a comparable unit in the area as determined by the Secretary under section 1437f of title 42 to a holder of a certificate of eligibility under that section solely because of such prospective tenant's status as a certificate holder