For the purposes of computing the total income under this Chapter, no deduction shall be allowed in respect of expenditure incurred by the assessee in relation to income which does not form part of the total income under this Act:

Provided that nothing contained in this section shall empower the Assessing Officer either to reassess under section 147 or pass an order enhancing the assessment or reducing a refund already made or otherwise increasing the liability of the assessee under section 154, for any assessment year beginning on or before the 1st day of April, 2001.