Both. Congress votes on bills, many of them related to spending, and the President can veto or sign those bills.
Many states have a similar setup with the state legislature voting on bills and the Governor having the ability to veto the bills. The fact that Gary Johnson as Governor of New Mexico took the state from debt to a one billion dollar surplus, mainly by vetoing spending bills, makes me believe that the President of the U.S. or Governors of states have much more control over spending then people might think.
Many states have a similar setup with the state legislature voting on bills and the Governor having the ability to veto the bills. The fact that Gary Johnson as Governor of New Mexico took the state from debt to a one billion dollar surplus, mainly by vetoing spending bills, makes me believe that the President of the U.S. or Governors of states have much more control over spending then people might think.