Separation of powers. Legislative & executive have different jobs. In this case, Congress passes the budge and authorizes borrowing, the President spends the $$$.
Checks and balances is, as you say, when one branch oversees how another branch does its job. Examples of c&b are the President signing/vetoing legislation, or the Senate approving/rejecting Presidential appointments and treaties.
Checks and balances violates the strict separation of powers, as is discussed at length by Madison in Federalist 47, 48 & 51.