Trump has total discretion in a national emergency.
Not to put too fine a point on it but president trump could declare a national emergency because somebody put pepperoni on his pizza and he could reallocate existing funds in the the trillions just to move that pepperoni. That’s exactly what Obama did and previous presidents... so guess what! this is one of those executive orders that they can repeal all they want to afterwords... it won’t remove the already buiklt walls.