"But the fact is we are both in a little bit of a political bind because I have to have Mexico pay for the wall – I have to.” -- Trump to Mexican President Enrique Peña Nieto on a January 27, 2017 phone call.
The Washington Post got its hands on Trump’s January phone conversations with Mexican President Enrique Peña Nieto and Australian Prime Minister Malcolm Turnbull. There's an extraordinary amount of insanity to be found in the transcripts, but the high point is probably Trump's pleading with with president Nieto to pay for The Wall, because he promised it to his voters.
From an article titled "Trump tried to pressure Mexican president on what to say about border wall, transcript reveals":
But in his first White House call with Mexico’s president, Trump described his vow to charge Mexico as a growing political problem, pressuring the Mexican leader to stop saying publicly that his government would never pay.
“You cannot say that to the press,” Trump said repeatedly, according to a transcript of the Jan. 27 call obtained by The Washington Post. Trump made clear that he realized the funding would have to come from other sources but threatened to cut off contact if Mexican President Enrique Peña Nieto continued to make defiant statements.
The funding “will work out in the formula somehow,” Trump said, adding later that “it will come out in the wash, and that is okay.” But “if you are going to say that Mexico is not going to pay for the wall, then I do not want to meet with you guys anymore because I cannot live with that.”