"it's not discretionary so Congress can't stop it."
Certainly in the UK Parliament ultimately controls the purse strings because they have to approve of taxation. If they really want to stop something they can, even if it means a revolt by MPs of the governing party against their leadership (i.e. the head of government).
The question for Congress would be whether it has the power to make something discretionary; as the elected body should it be able to make that decision. It might be something of a constitutional crisis if push came to shove but there seem to be reasonably frequent reports of Congress holding the government's feet to the fire by threatening to withhold the entire budget so it doesn't seem impossible.
Biting the hand that feeds IT
