What did I say when Trump first put forward The Wall for Mexico, and Mexico said "we won't pay"?
I said the solution is simple: Cut off remittances.
In the memo, Trump said he would threaten to change a rule under the USA Patriot Act antiterrorism law to cut off a portion of the funds sent to Mexico through money transfers, commonly known as remittances. The threat would be withdrawn if Mexico made “a one-time payment of $5-10 billion” to pay for the border wall, he wrote.
PS: Oh sure, there would be litigation. But it's litigation I suspect Trump would win. What would be even more amusing would be to both do it administratively (drawing said lawsuit) and then send up a bill to The House and ask them to pass it explicitly authorizing it. That would force Congress to go on the record -- either support illegal invaders as policy or pass the bill into law. If they pass the bill that moots the litigation at the same time...