The solution: Shut down Mexico's government until THEY fund the wall.

Mr. President,

You have said that we will have to finance the wall and then have Mexico pay for it. Let's cut out the middleman and just make Mexico pay for it up front. There would be a lot less bickering and it would be easier for accountants and so forth. And all we have to do is close down Mexico's government. There are probably many ways to do this (asking, pleading, threatening, screaming, holding hostages, etc.), but I'll leave that for your enormous brain to figure out.

After that it's clear sailing!

Respectfully,

skip fox