If part only of the lands charged with any such feu duty, ground annual, casualty of superiority, or any rent, payment, or incumbrance, be required to be taken for the purposes of the special Act, the apportionment of any such charge may be settled by agreement between the party entitled to such charge and the owner of the lands on the one part, and the promoters of the undertaking on the other part, and if such apportionment be not so settled by agreement the same shall be settled by the sheriff; but if the remaining part of the lands so jointly subject be a sufficient security for such charge, then, with consent of the owner of the lands so jointly subject, it shall be lawful for the party entitled to such charge to discharge therefrom the lands required, on condition or in consideration of such other lands remaining exclusively subject to the whole thereof.