Mark my words. Trump will use other funds to pay for this. It will be built, on time, on budget.
If Obama could write 500 million dollar checks to the UN to fight global warming in 3rd world countries,
http://www.politico.com/story/2017/01/green-climate-fund-obama-administration-233708
President Trump can do the same to get construction started on border wall security to protect Americans from a real and present threat.