Where a works contractor, in the course of execution of works contract, uses the goods supplied by the person for whom or for whose benefit a works contract is executed (hereinafter referred to as the principal), he may for the purpose of determining taxable contractual transfer price, deduct from the total contractual transfer price under clause (d) of sub-section (2) of section 18 that part of contractual transfer price which represents cost of such goods, supplied by the principal, the purchase of which have been made from within West Bengal on payment of tax under the Act.