of the Board of Investment of Sri Lanka Law, No. 4 of 1978, on or after April 1, 2000 but prior to the date of commencement of this Act and the agreement provides for the exemption of the whole or any part of the profits and income of such enterprise from income tax payable under the principal enactment, such exemption shall be deemed to have been, and to be, valid and effectual from the date of such agreement, as if such exemption had been expressly granted to the enterprise by a provision of the principal enactment.