(2A) 4[ Notwithstanding anything contained in sub- section (1A) of section 6 or sub- section (1)[ or clause (b) of sub- section (2)] 2[ of this section, the tax payable under this Act by a dealer on his turnover in so far as the turnover or any part thereof relates to the sale of any goods, the sale or, as the case may be, the purchase of which is, under the sales tax law of the appropriate State, exempt from tax generally or subject to tax generally at a rate which is lower than 2[ four per cent] (whether called a tax or fee or by any other name), shall be nil or, as the case may be, shall be calculated at the lower rate. Explanation.-- For the purposes of this sub- section a sale or purchase of any goods shall not be deemed to be exempt from tax generally under the sales tax law of the appropriate State if under that law the sale or purchase of such goods is exempt only in specified circumstances or under specified conditions or the tax is levied on the sale or purchase of such goods at specified stages or otherwise than with reference to the turnover of the goods.]