The appropriations clause says:
No money shall be drawn from the Treasury , but in Consequences of Appropriations made by law; and a regular Statement and Account of the Receipts and Expenditures of all public Money shall be published from time to time.
There is nothing that compels that the Executive Branch has to spend the money, but only that if it does it can only come from monies appropriated for it, and that it must be recorded and accounted for with receipts and made public.
Note: When the page fully loads the appropriations clause is at the top of the page.
Well, that certainly is more like it. But “discretion” is still limited by the Constitution.