As a matter of national pride neither the Mexican executive nor the Congress would entertain the idea of making this payment. They would have the broad backing of the Mexican people who widely object to Mr. Trump’s characterization of their compatriots living in the U.S. as rapists and drug dealers.
If Mexico refused to pay, under the Trump plan the U.S. government would invoke the revised Patriot Act to begin prohibiting aliens in the U.S. from wiring money unless they could prove that they are in the country legally.
A rewrite of the Patriot Act is the prerogative of Congress and an attempt by the executive to do it would be challenged in court and probably ruled illegal. But even if it were not, it is absurd to think that it will stop the flow of dollars to Mexico.
The most obvious way around the new provision would be for those who can prove legal residency to send the money for those who cannot. But wire transfers are only one way of moving money. It could also be deposited in a U.S. bank and withdrawn from automatic teller machines in Mexico by intended recipients.