The Congressional Budget Office is a federal agency within the legislative branch of the United States government. More at Wikipedia
United States Congress - The United States Congress is the bicameral legislature of the federal government of the United States of America, consisting of the Senate and the House of Representatives.
United States federal budget - The Budget of the United States Government is the President's proposal to the U.S. Congress which recommends funding levels for the next fiscal year, beginning October 1.
Office of Management and Budget - The Office of Management and Budget is a Cabinet-level office, and is the largest office within the Executive Office of the President of the United States.