(a) The tax in respect to motor vehicle fuel so sold or otherwise disposed of or used in any calendar month shall be paid by the importer on or before the 25th day of the next succeeding calendar month to the Collector of Taxes of the District of Columbia, who shall issue a receipt to the importer therefor.
(b) In the event a user obtains, sells, uses, or otherwise disposes of motor vehicle fuel in the District of Columbia upon which the tax imposed by this subchapter has not been paid, he shall be liable for the tax, penalties, and interest on such motor vehicle fuel as provided for in this subchapter.