No such contract, whereby the Commonwealth or any agency thereof or instrumentality of government shall undertake to purchase or acquire any slum area entered into before the completion of the clearance of such slum, shall be valid unless it shall impose a duty upon the Housing Authority to demolish, clear or remove all such buildings from the slum area as shall be necessary to change the character of said area and remove the slums therefrom as promptly as circumstances shall permit.
71 P.S. § 1676