Notwithstanding the provisions of this Act, it shall not be competent to the trustees to sell or lease part or parts only of the Peel Street property unless as a consequence of such sale or leasing an area of the improvements on such land that constitutes, in the opinion of the Governor in Council, not less than 51% of the commercially useable floor space in those improvements remains vested in or, as the case may be, in the exclusive possession of the trustees.