Determination of taxable turnover In determin ing the taxable turnover, the amounts specified in the following clauses, shall subject to the conditions specified therein, be deducted from the total turnover of the dealer 388 (a) all amounts allowed as discount, provided that such discount is allowed in accordance with the regular practice of the dealer or is in accordance with the terms of a contract or agreement entered into in a particular case and provided also that the accounts show that the purchaser has paid only the sum originally charged less the discount: (b) (i) all amounts allowed to purchasers in respect of goods returned by them within a period of 3 months from the date of delivery of the goods to the dealer when the goods are taxable on the amount for which they had been sold provided that the accounts show the date on which the goods were returned and the date on which and the amount for which refund was made or credit was allowed to the purchaser .