D.C. Mun. Regs. tit. 9, r. 9-410
The District sales tax is required to be reported and paid to the District on the basis of the full amount of the sale price of all property subject to tax.
Neither the amount of tax required to be reported and paid, nor the time for filing a return or paying the tax to the District is affected by the fact that sales were made on credit or on a deferred payment plan, or by the fact that the accounts receivable may subsequently prove to be uncollectible in whole or in part.
If a vendor sells or assigns its accounts receivable at a discount, those discounts may not be used to reduce the amount on which the tax is required to be paid, or to obtain a credit or refund.
A deduction shall be allowed from the gross receipts required to be reported in a tax return for any portion of any accounts that are or may prove to be uncollectible; Provided, that any amount so deducted which may thereafter be collected shall be included without a reduction for collection expense in the first return after such collection.
D.C. Mun. Regs. tit. 9, r. 9-410