D.C. Code § 47-1308
The Mayor of the District of Columbia shall not convey any property sold for taxes if he shall discover, before the conveyance, that the sale was for any cause invalid and ineffectual to give title to the property sold; but he shall cancel the sale and cause the purchase money, together with interest at the rate of 6% per annum, and the surplus, if any, to be refunded to the purchaser, his representatives or assigns; provided, that if any conveyance made by the Mayor, of property sold for taxes, shall at any time be set aside by decree of any court as invalid, the party in whose favor the decree is rendered shall pay to the party holding such conveyance, his heirs or assigns, the amount paid for such taxes and conveyances, together with interest at the rate of 6% per annum.
D.C. Code § 47-1308