D.C. Mun. Regs. tit. 9, r. 9-412
The reimbursement for the tax to be collected by the vendor from the purchaser shall be stated separately from the sales price of the property sold or services rendered.
In the case of a vendor not engaged in business in the District who makes sales at retail and who has been expressly authorized to pay the tax imposed by this title, that vendor shall, at the time of making such sales, collect reimbursement for the tax from the purchaser and give to the purchaser a receipt which shows separately the sales price of the property sold or service rendered and the reimbursement for the tax.
If a vendor collects reimbursement for the tax from the purchaser in excess of the amounts prescribed in § 413, or if the vendor collects reimbursement for the tax on any exempt or non-taxable sale or charge, the vendor shall make a refund or credit to the purchaser for the amount of the overcharge.
To the extent that each overcharge has not been refunded or credited to the purchaser, it shall be paid to the District in the same manner and at the same time as the tax required to be paid by the provisions of the Act (D.C. Code § 47-2016) .
The gross receipts from any sales covered by § 414 shall be excluded from the total gross receipts of the seller making such sales and the seller shall neither collect nor add any reimbursement for tax to the consideration charged for such retail sales.
D.C. Mun. Regs. tit. 9, r. 9-412