(1) No tax shall be payable under this Act on the sale of goods specified in the Third Schedule subject to the conditions and exceptions, if any, set out therein.
     (2) The Administrator may, with the previous approval of the Central Government and by notification in the Official Gazette, add to, or omit from, or otherwise amend, the Third Schedule either retrospectively or prospectively, and thereupon the Third Schedule shall be deemed to be amended accordingly:
     Provided that no such amendment shall be made retrospectively if it would have the effect of prejudicially affecting the interests of any dealer.