D.C. Mun. Regs. tit. 9, r. 9-416
It shall be unlawful for any vendor to advertise, hold out, or state to the public or to any customer, directly or indirectly, that the reimbursement for the tax or any part of the tax to be collected by the vendor under this chapter will be assumed or absorbed by the vendor.
It shall be unlawful for any vendor to advertise, hold out, or state to the public or to any customer, directly or indirectly, that the reimbursement for the tax or any part of the tax to be collected by the vendor under this chapter will not be added to the selling price of the property sold or the taxable services rendered; or if added to the sales price, that the tax (or any part of it) will be refunded.
Any person violating any provision of this section shall, upon conviction, be fined not more than five hundred dollars ($ 500) or imprisoned for not more than six (6) months, or both, for each offense.
D.C. Mun. Regs. tit. 9, r. 9-416