In his first prime-time news interview since assuming the Presidency, Donald Trump tells David Muir that the border wall he promised between the United States and Mexico will begin construction in months. As for who will be paying the bill, that answer is a little more complicated.

From ABC News:

“Ultimately, it will come out of what’s happening with Mexico … and we will be in a form reimbursed by Mexico, which I’ve always said,” Trump said.

During the interview, which took place at the White House this morning, Trump said that Mexico would pay the U.S. back “100 percent.”

He confirmed that U.S. taxpayer dollars would be used to start the construction but said reimbursement would follow.

“All it is, is we’ll be reimbursed at a later date from whatever transaction we make from Mexico,” he said. “I’m just telling you there will be a payment. It will be in a form, perhaps a complicated form. What I’m doing is good for the United States. It’s also going to be good for Mexico. We want to have a very stable, very solid Mexico.”

When asked about the start of construction, Trump said it would happen in “months.”

“As soon as we can, as soon as we can physically do it,” he said. “I would say in months, yeah. I would say in months — certainly planning is starting immediately.”

It’s not yet entirely clear how Mexico will “reimburse” the United States for the wall, though there have been many theories and proposals floated since Trump started his “build the wall” rhetoric. There could be import taxes or perhaps a tax on outgoing money orders to Mexico, that part isn’t settled yet. Regardless of what happens, this will be a huge issue moving forward in Trump’s presidency since it is one of his signature campaign promises. His voters and supporters will be watching closely.

The full hour long interview will air tonight at 10pm ET on ABC.