https://thehill.com/opinion/white-ho...-likely-to-win
It is true that Congress has power of the purse, but it has given the president a fair amount of powers on national emergencies and its funding thereof.
As of 1973, congress had passed more than 470 statutes granting national emergency powers to the president. National emergency declarations under those statutes were rarely challenged in court.
50 U.S.C 1601 1976 National Emergencies Act (NEA)
The president must:
Specify the provisions of law under which he intends to act;
Transmit the declaration to congress immediately and publish it in the Federal Register;
Maintain a file and index of orders, rules, and regulations issued pursuant to the declaration;
Provide reports to congress on expenditures directly attributable to the declaration.
10 U.S.C. § 2808 emergency military construction authority
33 U.S.C. 2293 gives the president the authority to reprogram certain civil defense funds
Secure Fence Act of 2006 (Public Law 109-367)
there are a few more of those laws Trump can use in conjunction with the NEA.
http://thefederalist.com/2019/02/19/...cy-build-wall/
This is a legal analysis of Trump's move.
In the absence of federal laws prohibiting the deployment or use of the armed forces (e.g., the Posse Comitatus act prohibits the Army and Navy from engaging in domestic law enforcement), the president’s discretion on whether use of the armed forces is required to secure and defend the United States is absolute.
Similar to the law authorizing the president to declare a national emergency, nowhere in the statute at issue does Congress set forth certain conditions that must be met prior to determining that the use of the U.S. military is required.
Trump is operating within the bounds of a clearly defined and narrow delegation of authority within the realm of appropriations law.