"Mexico paying for the wall" was not the main promise. And he already laid out that his plans on making "Mexico pay" was to pull out of what he considered to be bad trade deals and such. He can quiver out of making Mexico pay, but he cannot quiver out of building a wall.
Trump's voters won't give a damn whether Mexico directly pays for the wall or not. They just want it built. The only people who care about Mexico paying for it, are anti-Trumpers, who ultimately aren't Trump's concern, since they will never vote for him or support him anyway, regardless of what he does. His actual voter base is the priority, and will remain as such, often to the chagrin of those who did not vote for him.
You can have your little arguments over the details of it, arguing semantics, but this is the reality of it, the reality that Trump is facing. He needs to get the wall built. To save his own face, and to save the face of American democracy, for the people that voted for him.
Ineffeciency in that regard will not be forgiven, even if the process of getting the wall built ended up being different from how it was originally spelled out, in the heat of campaigning. All prior promises by presidential candidates, usually ended up being half-assed compromises, too. Nobody really "gets their way" in that regard.
