The President can move money around to pay for the wall, that can be done via an Executive Order because he has been ordered by Congress to do so. He just takes out of discretionary funds, no problem.
If we can send $221 Million dollars to Palestinian Authority without Congressional Authorization. If the President can fly a billion dollars in cash to Iran or release 12 billion to Iran, he can find the money to build the wall.
“Congressional aides say there is about $100 million of unspent appropriations in the Department of Homeland Security account for border security, fencing and infrastructure. That would allow planning efforts to get started”
He can basically do what he wants, unless Congress steps in and stops him.