Provided always, That the Restrictions herein-before contained against taking any Fine or Consideration in the Nature of a Fine on the granting of Leases of the Possessions of the said Duchy shall not apply to or affect any Disposal for a limited Period, in consideration wholly or in part of a gross Sum of Money, of any Part or Parts of the Possessions of the said Duchy which may be made in any special Case under the Powers herein-before contained, with such Sanction and Approval of the Lord High Treasurer or of the Commissioners of the Treasury as herein-before is mentioned, and under which the gross Sum received as the whole or in part of such Consideration will be treated as Capital Money.