D.C. Mun. Regs. tit. 9, r. 9-409
A vendor making any sale on credit shall report the total sale price of the property sold on the return covering the month in which the sale was made and pay the tax on the sale.
The term "sale on credit" includes, but is not limited to, the following:
If a sale is made on credit, lay-away, budget plan, or other deferred payment plan, the gross sales price becomes taxable when the sale, agreement or other arrangement for transfer of the property from the vendor to the purchaser is made. The vendor shall collect reimbursement for the tax on the total sales price at the time the first payment is made by the purchaser.
If the purchaser returns the property to the vendor upon recision of the contract of sale within ninety (90) days of the date of sale and the vendor allows the purchaser credit or cash on the full amount of the sales price charged for the property (including the sales tax reimbursement), the vendor is entitled to a refund of the tax paid; Provided, that the vendor files an application for refund under oath with the Deputy Chief Financial Officer within three (3) years from the date of payment.
No deduction or refund will be allowed if any property sold on credit is repossessed by the seller or the seller's agent, representative, or assignee.
D.C. Mun. Regs. tit. 9, r. 9-409