The US Congress has budgeting authority. Says so in the constitution. The only fair way to do this sort of thing is hammer it out in Congress. Otherwise, it's just an exercise in deploying power, and people get upset. It's the other side of "no taxation without representation".

If the executive branch just up and does this as an exercise im dominance, it gives everyone whose metaphorical ox got hired the idea that they, too, can exercise dominance and get what they want.