D.C. Mun. Regs. tit. 9, r. 9-903
Each carrier shall be entitled to a credit equivalent to the total amount of tax per gallon on all motor fuel purchased by that carrier within the District and used in operations either within or outside the District, and upon which the motor fuel tax imposed under the District of Columbia Motor Fuel Tax Act has been paid.
In support of the credit claimed, the carrier shall provide evidence of the payment of the tax paid on motor fuel purchased in the District consisting of invoices of the vendor satisfactory to the Deputy Chief Financial Officer.
The original invoice shall be required; Provided, that a copy of an invoice may be used in case of credit card purchases.
In order to further substantiate the credit claim, the Deputy Chief Financial Officer is authorized to require a sworn affidavit from the vendor stating that the vendor has sold the specified number of gallons of motor fuel to the carrier and that the motor fuel tax on the fuel has been paid.
If the amount of the credit to which any carrier is entitled for any month exceeds the amount of the motor fuel tax for which that carrier is liable for the same month, the Deputy Chief Financial Officer shall refund the excess.
D.C. Mun. Regs. tit. 9, r. 9-903