At a rally in New Hampshire on Monday, he offered something resembling an explanation.
Facts First: Experts on immigration policy told CNN they had no idea what the term "redemption money" means. Trump is paying for the wall with a combination of US taxpayer money Congress has allocated to the project and funding he is diverting from the military.
"I don't know what the President means by redemptions. I have never heard the word used in this context before," said Stephen Yale-Loehr, a professor of immigration law practice at Cornell University. "In any event, the President is wrong. Money appropriated by Congress and money diverted from the Department of Defense is paying for construction of the wall, not immigrants."
Four Migration Policy Institute researchers focused on the United States have also never "heard of the use of the term 'redemption' in the immigration context," said Michelle Mittelstadt, the think tank's communications director.
Mittelstadt said the researchers surmised Trump might have meant to refer to remittances, money that immigrants send back to people in their countries of origin, but she emphasized that they weren't sure.
Theresa Cardinal Brown, director of immigration and cross-border policy at the Bipartisan Policy Center, also said Trump could perhaps have been thinking of remittances and misspoke, though she too said she doesn't know -- and that Trump might not know himself.
Regardless, Brown said, there is no magical mechanism, related to remittances or anything else, by which Mexico is paying for the wall.
"I have absolutely no idea what he means by that. Clearly the US is paying for the wall," she said.
The White House and the Trump campaign did not respond to requests for an explanation of "redemption money."
Trump's team has previously invoked remittances in the context of wall funding.
But none of these proposals was implemented.