Saying Mexico will pay for the wall through supposed USA gains in a trade pact, or by gains in any newly negotiated Mexico-USA cooperative deal, is just yet another
fallback position Trump admirers have to take when it is obvious Trump's original conning promise will never be kept. Fact is it was a blatant lie from the start.
Trump just said it because it served as honey to a gullible voter.
It was Plato who said, “He, O men, is the wisest, who like Socrates, knows that his wisdom is in truth worth nothing”