Whoever, being an officer, or employee of the United States or any department or agency thereof, or representing himself to be or assuming to act as such, under color or pretense of office or employment commits or attempts an act of extortion, shall be fined under this title or imprisoned not more than three years, or both; but if the amount so extorted or demanded does not exceed $1,000, he shall be fined under this title or imprisoned not more than one year, or both.