Section 49. If an urban renewal agency shall sell or lease any property acquired by it for an urban renewal project, the terms of such sales or leases shall obligate the purchasers or lessees, (a) to devote the land to the use specified in the urban renewal plan for said land; (b) to begin the building of their improvements within a reasonable time; provided, however, that, with respect to any improvements of a type which any federal agency, as defined in subsection (b) of section 3 of the Federal Property and Administrative Services Act of 1949, as amended, is otherwise authorized to make, this clause shall apply to such federal agency only to the extent that it is authorized, and funds have been made available, to make the improvements involved; (c) to give preference in the selection of tenants for dwelling units built in the project area to families displaced therefrom because of clearance and renewal activity who desire to live in such dwelling units and who will be able to pay rents or prices equal to rents or prices charged other families for similar or comparable dwelling units built as a part of the same redevelopment; and (d) to comply with such other conditions as are deemed necessary to carry out the purposes of this chapter, or requirements of federal legislation or regulations under which loans, grants or contributions have been made or agreed to be made to meet a part of the cost of the project. Nothing in this chapter shall be construed as limiting the power of an urban renewal agency in the event of a default by a purchaser or lessee of land in an urban renewal project to retake title to and possession of the property sold or leased free from the obligations in the conveyance or lease thereof.