(2) Any deduction allowable under the provisions of sections 30 to 38 shall, for the purposes of sub- section (1), be deemed to have been already given full effect to and no further deduction under those sections shall be allowed: Provided that where the assessee is a firm, the salary and interest paid to its partners shall be deducted from the income computed under sub- section (1) subject to the conditions and limits specified in clause (b) of section 40.