The section 44AD itself starts with........... 44AD. (1) Notwithstanding anything to the contrary contained in sections 28 to 43C, in the case of an eligible assessee engaged in an eligible business, a sum equal to eight per cent of the total turnover or gross receipts of the assessee in the previous year on account of such business or, as the case may be, a sum higher than the aforesaid sum claimed to have been earned by the eligible assessee, shall be deemed to be the profits and gains of such business chargeable to tax under the head "Profits and gains of business or profession" :
So, anything stated in section 40(b) not applicable to section 44AD.. Earlier the deduction was allowed from the deemed profit of 6/8% (vide sec, 44AD(2)) which has has been specifically removed by finance act, 2016.
The provision specified that the deductions u/s. 30 to 38 have deemed to have taken effect, but not 40(b), so if any deduction u/s. 40(b) taken by firm, it will be reversed !!!
So, the amount deducted by the partners can be tax-exempt capital, but not salary or interest!!! (when already 9% profit declared).