Congress's Power Over Appropriations: Constitutional and Statutory Provisions [June 16, 2020] [open pdf - 2MB]
From the Summary: "A body of constitutional and statutory provisions provides Congress with perhaps its most important legislative tool: the power to direct and control federal spending. Congress's 'power of the purse' derives from two features of the Constitution: Congress's enumerated legislative powers, including the power to raise revenue and 'pay the Debts and provide for the common Defence and general Welfare of the United States,' and the Appropriations Clause. This latter provision states that 'No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law.' Strictly speaking, the Appropriations Clause does not provide Congress a substantive legislative power but rather constrains government action. But because Article I vests the legislative power of the United States in Congress, and Congress is therefore the moving force in deciding when and on what terms to make public money available through an appropriation, the Appropriations Clause is perhaps the most important piece in the framework establishing Congress's supremacy over public funds."
CRS Report for Congress, R46417
Congressional Research Service: https://crsreports.congress.gov/