51. Advance money received—Where any capital asset was on any previous occasion the subject of negotiations for its transfer, any advance or other money received and retained by the assessee in respect of such negotiations shall be deducted from the cost for which the asset was acquired or the written down value or the fair market value, as the case may be, in computing the cost of acquisition :

Provided that where any sum of money, received as an advance or otherwise in the course of negotiations for transfer of a capital asset, has been included in the total income of the assessee for any previous year in accordance with the provisions of clause (ix) of sub-section (2) of section 56, then, such sum shall not be deducted from the cost for which the asset was acquired or the written down value or the fair market value, as the case may be, in computing the cost of acquisition.