The Secretary may enter into agreements, or authorize housing owners to enter into agreements, with public or private agencies for services required in the selection of qualified tenants, including those who may be approved, on the basis of the probability of future increases in their incomes, as lessees under an option to purchase (which will give such approved qualified tenants an exclusive right to purchase at a price established or determined as provided in the option) dwellings, and in the establishment of rentals. The Secretary is authorized (without limiting his authority under any other provision of law) to delegate to any such public or private agency his authority to issue certificates pursuant to this subsection.