Are we making the Omnibus Spending Bill/Wall Funding more complicated than it actually is?

The Department of Defense got $654.6 billion in the Omnibus Spending Bill. POTUS is Commander in Chief of the armed forces and the various support units in the DoD. If POTUS tells the Army Corp of Engineers that he feels that building a wall will enhance our nation's security by establishing a clearly defined, impenetrable border I think the head of the Army Corp of Engineers will say "Yes, Sir!", and build him a "big, beautiful wall".