(a) The following items are not included in gross income:
(1) Cash discounts allowed by the vendor for timely payment, but only discounts allowed against taxable gross income.
(2) The value of property returned by customers to the extent of the amount actually refunded either in cash or by credit and the amount refunded was included in taxable gross income.
(3) The trade-in allowance for tangible personal property accepted as payment, not to exceed the full sales price for any tangible personal property sold, when the full sales price is included in taxable gross income. Trade-in allowances are not allowed for manufactured buildings taxable under Section 12-427.
(4) When coupons issued by a vendor are later accepted by the vendor as a discount against the transaction, the discount may be excluded from gross income as a cash discount. Amounts credited or refunded by a vendor for redemption of coupons issued by any person other than the vendor may not be excluded from gross income.
(5) Rebates issued by the vendor to a customer as a discount against the transaction may be excluded from gross income as a cash discount. Rebates issued by a person other than the vendor may not be excluded from gross income, even when the vendee assigns his right to the rebate to the vendor.
(6) In computing the tax base, gross proceeds of sales or gross income does not include a manufacturer’s cash rebate on the sales price of a motor vehicle if the buyer assigns the buyer’s right in the rebate to the retailer.
(b) If the amount specified in subsection (a) above is credited by a vendor subsequent to the reporting period in which the original transaction occurs, such amount may be excluded from the taxable gross income of that subsequent reporting period, but only to the extent that the excludable amount was reported as taxable gross income in that prior reporting period.