Still don't get how people don't understand this issue. The man's been extremely successful in RED (real estate development).
You usually don't get a contractor that does shoddy work/failed to uphold his terms of the agreement, to pay back money. They refuse, use legal loopholes (at least overseas), have various methods as well to finagle out of a deal...and because you are a smart RED, and never pay full amounts at the onset. Payment goes through in contracted stages, after meeting certain benchmarks, from architects and planners at the design process, to furniture procurement and interior installations. If you don't do good work, your future payment is withheld, and there are many other areas (lawsuits, or withholding payment for other services to the same company, ) to recoup the loss. You don't necessarily get that money directly from them. You get it back in other ways.
All over the developed world, it goes like this. Standard RED procedure. And as a nation-state, the US has a LOT more options to recoup the expenses, especially from a country that a) is right next door and b) in ages past would have been conquered or annihilated for its wrongdoings, if the US was anything like earlier civilizations.
I don't think anyone would assume Mexico will "pay" for the wall by providing us some sort of check, or truckloads of greenbacks or something. What will happen is, construction on the "wall" (fencing, drones, increased budget for border security, reduction of funding to sanctuary cities, deportation of illegals, freezing bank accounts, etc. etc.) will start with taxpayer funding, even be completed with a majority using tax dollars. I've always thought this would be the case. Devs/investors always cover initial costs in a development project, then accrue funding, and payout, accordingly.
US will ask Mexico for recompense and funding after the project is started, whether via materials, currency, or other measures. Refusal? Then there could be potential limits / tariffs on Mexican exports to the US, limitations and barriers to currency transfers, cutback on types of aid, withdrawal from various cooperative bodies, etc., to a point where Mexico has "paid" for the costs.