I don’t buy that executive discretion canard especially when it excuses liberal spending. Faithful execution of the laws does not entail entertaining a chief executive’s whim, but within the bounds of law. Same goes for Congress in its appropriation of money.
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.