There are really two questions involved in the decision about emergency funding. First, does statutory law allow for the president’s emergency orders, and, second, does the Constitution permit these emergency orders? As far as the statute goes, the answer is maybe — although no president has previously used emergency powers to spend money denied by Congress, and it was clearly not intended to do that.
But there is a much larger question: the question of whether or not this power and therefore this action are constitutional. With regard to the Constitution, the Supreme Court made it very clear in Youngstown Steel in 1952, in a case that is being closely reexamined in the discussion of executive power. In Youngstown, the Court ruled that there are three kinds of executive order: orders that carry out an expressly voiced congressional position, orders where Congress’ will is unclear, and, finally, orders clearly opposed to the will of Congress.
To my mind, like it or not, we had this conversation. In fact, the government was shut down in a public battle over how much money would be spent on the wall and border security. It ended with a deal that Congress passed and the president signed into law, thus determining the amount.