law: The Black Money (Undisclosed Foreign Income and Assets) and Imposition of Tax Act, 2015
  5. Computation of total undisclosed foreign income and asset.-
  (1) In computing the total undisclosed foreign income and asset of any previous year of an assessee,-
  (i) no deduction in respect of any expenditure or allowance or set off of any loss shall be allowed to the assessee, whether or not it is allowable in accordance with the provisions of the Income-tax Act;
  (ii) any income,-
  (a) which has been assessed to tax for any assessment year under the Income-tax Act prior to the assessment year to which this Act applies; or
  (b) which is assessable or has been assessed to tax for any assessment year under this Act, shall be reduced from the value of the undisclosed asset located outside India, if, the assessee furnishes evidence to the satisfaction of the Assessing Officer that the asset has been acquired from the income which has been assessed or is assessable, as the case may be, to tax.
  (2) The amount of deduction referred to in clause (ii) of sub-section (1) in case of an immovable property shall be the amount which bears to the value of the asset as on the first day of the financial year in which it comes to the notice of the Assessing Officer, the same proportion as the assessable or assessed foreign income bears to the total cost of the asset.
  Illustration
  A house property located outside India was acquired by an assessee in the previous year 2009-10 for fifty lakh rupees. Out of the investment of fifty lakh rupees, twenty lakh rupees was assessed to tax in the total income of the previous year 2009-10 and earlier years. Such undisclosed asset comes to the notice of the Assessing Officer in the year 2017-18. If the value of the asset in the year 2017-18 is one crore rupees, the amount chargeable to tax shall be A-B=C
  where,
  
  A=Rs.1 crore, B=Rs. (100 x 20/50) lakh= Rs.40 lakh, C=Rs. (100-40) lakh=Rs.60 lakh.