After a bit of googling I found this link which I assume summarizes the history of this correctly. Back in 2016, when called on specifics for how Mexico would pay for the wall, Trump offered a number of ways to make that happen, through this or that form of economic pressure on Mexico, but "making a new and better trade deal with Mexico" didn't appear to be among them. If he had made that suggestion back then, then at least there would be some consistency across time by what he meant by Mexico paying for the wall (as dubious as that sounds in any case). But it was only recently that he started with the "The new trade deal will pay for the wall (several times over)" explanation.
This is quite symptomatic of how the orange man makes factual claims and how slippery he gets when called out on the more dubious of them. In ordinary understanding, paying for X means that there is a tit-for-tat exchange of the payment for X, an exchange that won't take place unless the payment happens. Obviously, of Mexico really were paying for the wall, it would be making a payment for X and not getting X in return; rather, the recipient of the payment would be getting the (benefits of) X.
Basically, what Trump was promising was that Mexico would engage in a "deal" (the art of which Trump claims to have mastered if not perfected) in which it gives up both the payment and the benefit of it to the other party. That sounds more like the deal that the Empire cut with Lando in The Empire Strikes Back (the best punchline being Vader: "I'm altering the deal, and pray I don't alter it further"), not anything resembling a mutually-beneficial deal that Trump claims to be an expert at making.
The benefits of a trade deal are fungible; it makes just as much sense to say that the overall benefit of the deal will go to paying for, among other things, $5B worth of diapers for American babies. There: Mexico is paying for American diapers, and they're doing it via the new trade deal. But they still need to pay for the wall, after they've paid for the diapers and lots of other stuff benefiting Americans that the deal is paying for. So when is that payment for the wall going to happen?
This is where common sense has to take over and make light of the fact that it was a bullshit promise that he could, at the very least, have been consistent on over time. Had he made clear back in 2016 that "negotiating a better trade deal with Mexico would be a way to pay for the wall," then that's what the American people would have to go on in assessing his credibility back then.
It appears (as common sense would have it) that Mexico wasn't going to pay for the border wall unless it was going to get at least as much of a benefit in return for that payment. Having faced this reality, Trump had to think and think and think, in the wee hours of the morning between tweets, for months and months on end, how he was going to "make good" on that promise, and this seems to be the best he can come up with after all that thinking. What, you're not impressed?
