Whoever, being an officer, employee or agent of the United States or of any department or agency thereof, having received public money which he is not authorized to retain as salary, pay, or emolument, fails to render his accounts for the same as provided by law is guilty of embezzlement, and shall be fined under this title or in a sum equal to the amount of the money embezzled, whichever is greater, or imprisoned not more than ten years, or both; but if the amount embezzled does not exceed $1,000, he shall be fined under this title or imprisoned not more than one year, or both.