No such contract to lease, grant, assign or convey any slum area to the Commonwealth, or any of its agencies, or instrumentality of government, including the General State Authority, entered into before the clearance of such slums, shall be valid unless it shall reserve the right unto and 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 permit.