Article I, Section 1 of the Constitution consists of this single sentence. I have given you that, plus a modern usage, cut to the chase version. The power to make laws belongs solely to Congress.
One could ask then, what the President is doing putting together a line item spending proposal and asking Congress to approve it? It turns out that Congress was the one to require that action through the Budget and Accounting Act of 1921. Under that act the Bureau of Budget, now the Office of Management and Budget, was created to assist the President in producing the budget required of him. It reports directly to the President. It also created the Goverment Accountability Office to provide independent audit and oversight of government accounts for Congress. However, it was not until the Congressional Budget and Impoundment Control Act of 1974 that the Congressional Budget Office was created giving Congress an independent view of budgetary and economic issues.