D.C. Code § 28-3305: Unlawful interest credited on principal debt.

In an action upon a contract for the payment of money with interest at a rate forbidden by law, any payment of interest that may have been made on account of the contract is deemed to be payment made on account of the principal debt; and judgment shall be rendered for no more than the balance found due after deducting and properly crediting the interest so paid. A bona fide indorsee of negotiable paper purchased before due is not affected by any usury exacted by a former holder of the paper unless he had notice of the usury before his purchase.