If a registered dealer or any other dealer contravenes the provisions of section 64, the Commissioner may, after giving such dealer a reasonable opportunity of being heard, by an order in writing, direct that he shall pay,in the manner as may be prescribed, by way of penalty, a sum equal to double the amount of tax or Ten thousand rupees, whichever is greater, which could have been levied under this Act in respect of the sales referred to in that section where-

(a) no tax invoice or invoice or cash memorandum or bill, as the case may be, as referred to in section 64, has at all been issued, or

(b) tax invoice or invoice or cash memorandum or bill, as the case may be, has been issued not in accordance with the provisions of section 64 :

Provided that if such registered dealer or dealer proves to the satisfaction of the Commissioner that he deals exclusively in goods specified in Schedule A, sales of which are declared tax-free under section 21, the Commissioner may exempt such registered dealer or dealer from payment of penalty or impose such lesser amount of penalty as he deems fit and proper.