It shall be lawful for the promoters of the undertaking to sell the lands which they shall have so acquired for extraordinary purposes, or any part thereof, in such manner, and for such considerations, and to such persons, as the promoters of the undertaking may think fit, and again to purchase other lands for the like purposes, and afterwards sell the same, and so from time to time; but the total quantity of land to be held at any one time by the promoters of the undertaking for the purposes aforesaid shall not exceed the prescribed quantity.