The agent of each tribe of Indians, lawfully residing in the Indian country, is authorized to sell for the benefit of such Indians any cattle, horses, or other livestock belonging to the Indians, and not required for their use and subsistence, under such regulations as shall be established by the Secretary of the Interior. But no such sale shall be made so as to interfere with the execution of any order lawfully issued by the Secretary of the Army, connected with the movement or subsistence of troops.