One of the ploys, used by Donald Trump, to convince his supporters, to vote for him, was that, because he was, himself, guilty of gaming the system, he would be the ideal person to be able to use his skills to benefit his supporters.
One of his skills has been to use other people's money to build things to his benefit and then to manipulate circumstances so as to avoid paying for them either partially or in full. That is why it seemed perfectly feasible, to some, that he could build a multi-billion dollar wall, which could accrue to the benefit of America, without having to pay for it. Now it appears that the American taxpayer is to advance the funds and Trump will somehow have a 3d party reimburse us by an undisclosed method, just "trust him".
In the past, it has been the people who have advanced the funds, or in some cases,the goods and services, that have been left holding the bag. Trump has a lot of experience in that sort of a scenario; not so much in paying funds back.