D.C. Code § 13-505: Action by or against executor or administrator.

In an action against an executor or administrator, in his representative capacity, the defendant may plead, by way of counterclaim, a demand belonging to the decedent where he would have been entitled to rely upon the demand in an action against him; and in an action brought by an executor or administrator, in his representative capacity, a demand against the decedent, belonging at the time of his death to the defendant, may be pleaded by way of counterclaim, as if the action had been brought by the decedent in his lifetime.