Neither the Department nor any employee of the Department shall comply with any demand for production of material or provision of testimony in a legal proceeding in which the United States is a party, unless the prior approval of the attorney representing the United States has been obtained in accordance with this subpart. This rule does not apply to any legal proceeding in which an employee may be called to participate, either through the production of documents or the provision of testimony, not on official time, as to facts or opinions that are in no way related to material described in § 15.301.