Except for the official handling, through the proper channels, of matters relating to legislation in which the Department has an interest, employees shall not use government time, money, or property to petition a Member of Congress to favor or oppose any legislation or proposed legislation, or to encourage others to do so. This section does not prohibit the use of government time by union representatives to petition a Member of Congress to favor or oppose any legislation or proposed legislation, where permitted by the terms of a collective bargaining agreement.