(a) on all his sales in West Bengal of goods brought by him from any place outside West Bengal either by way of purchase from a person, or procured by him otherwise, and

(b) on his every purchase of goods in West Bengal, after deducting therefrom--

(i) purchases of goods, sales of which are declared tax-free under section 21;

(ii) purchases which are shown to the satisfaction of the Commissioner to have taken place in the course of inter-state trade or commerce within the meaning of section 3 of the Central Sales Tax Act, 1956, or in the course of import of goods into, or export of the goods out of the territory of India within the meaning of section 5 of that Act;

(iii) purchases of goods which are shown to the satisfaction of the Commissioner to have been made from a registered dealer in West Bengal: Provided that the burden of proving that any purchase effected by the casual dealer is not liable to tax shall be on such casual dealer.