If a person is not legally entitled to be in America, he is not legally entitled to work in America. Ergo, any monies that person receives from his labor would be "illegal wages."
To impound all of these remittance payments (a remittance payment happens when a foreign national sends money home to his or her family in his or her country of origin) he would have to start by intercepting and opening any letter addressed to Mexico to remove any money contained therein.
Next, he'd have to commandeer the worldwide electronic funds transfer industry to shut down both direct US-Mexico wire transfers and "cutout" transfers - Jorge in Los Angeles wires funds to Mr. Park in Seoul who wires them to Samantha in Montego Bay, Jamaica, who wires them to Gabriel in Mexico City.
Once that was closed up, he would then have to station people in every international airport in Mexico and every place in Mexico where you could land an ocean-going boat to search every person entering Mexico for evidence they're carrying money in.
Problem 1: this WOULD start a war on our southern border. Which we would lose. Badly.
Problem 2: I don't know how many trillions of dollars we spent over the last 70 years to arm ourselves against guys named Erich Honecker, Wojciecz Jaruzelski, Josip Tito, Nicolae Ceaucescu, Kim Il Sung, Mao Tse Tung, Ho Chi Minh, Leonid Brezhnev, Fidel Castro, and Saddam Hussein who were doing this exact same shit, but it was enough to convince me we don't need to go down the same road.
and
Problem 3: It wouldn't work anyway. There are WAY too many ways to move money from place to place the Trumpster can't detect, including putting ten million dollars of remittance payments (which you would carry for a modest 10 percent commission) in a stateroom on your 60-foot Hatteras Yacht and heading out in the middle of the night to fish for man-eating sharks off the coast of Mexico.
After seeing this kind of thing, I have to wonder why a communist like Donald Trump loves capitalism so much.