In any case to which this section applies, the wholesaler or manufacturing retailer, as the case may be, shall, in the return furnished by him in accordance with section 14 of the principal Act, be entitled to deduct from the sales tax payable on goods sold or manufactured by him during the month to which the return relates the amount of sales tax originally paid by a wholesaler, manufacturing retailer, or importer in respect of any such goods. The amount that may be deducted under this subsection shall be deemed to include the amount of any discount lawfully deducted from the last-mentioned sales tax on its payment, but not to include any additional sales tax added to it as a penalty for late payment.”