D.C. Mun. Regs. tit. 9, r. 9-406
In addition to the provisions of the Act, (D.C. Code § 47-2001(p)(2) ), the term "sales price," as used in the Act, shall not include any of the exceptions set forth in this section.
For the purposes of D.C. Code § 47-2001(p)(2)(A), if a vendor quotes to a purchaser a list price with a deduction from that price for a trade discount, the tax shall be computed on the list price less the trade discount.
A trade discount shall be an expedient for adjusting a list price and is to be deducted in arriving at the true selling price of the property.
A quantity discount is a trade discount allowed where purchases equal or exceed a predetermined amount and may be excluded from the selling price only when allowed currently.
No credit or refund may be claimed where quantity discounts are allowed subsequent to the sale to which the discount is applicable.
If tangible personal property shall be sold subject to a discount for payment within a limited time, the tax applies to the total selling price of the property without any allowance for the discount.
In the case of cash sales where the discount is deducted from the selling price and the net amount due on the sale is paid at the time of sale, the sales price shall not include the discount.
D.C. Mun. Regs. tit. 9, r. 9-406