Problem solved! Photo: Kris Connor/Getty Images

President Trump just fulfilled his biggest campaign promise, signing an executive order on Wednesday that orders the “immediate construction of a physical wall” between the United States and Mexico. Now the Trump administration just needs to work out the massive logistical challenges and figure out how to pay for the wall, which could cost as much as $20 billion.

On the campaign trail, Trump repeatedly said that Mexico would pay for the wall, but in recent weeks he and other Republican leaders began emphasizing that U.S. taxpayers would actually pay for the wall at first, and Mexico would reimburse them.

Many noted on Wednesday that despite Trump’s order, the government can’t spend money that Congress hasn’t appropriated. But House Speaker Paul Ryan confirmed in an interview with MSNBC’s Greta Van Susteren on Wednesday that Congress is going to find a way to let Trump use U.S. taxpayer money to kick off construction of the wall.

“First off, we’re going to pay for it and front the money,” Ryan said. He didn’t elaborate, but according to CNN, during the Republican lawmakers’ retreat in Philadelphia this week, Ryan said they could push through a special spending bill dedicated to the wall in the next few months.

“[Ryan] talked about the supplemental budget coming this spring to push that along. The president said he’s going to get reimbursed one way or the other and we accept that,” said Representative Chris Collins. “Meanwhile, we do have to pay our bills.”

It’s unclear where the funding for that measure would come from, but of course, that’s only a temporary concern. Ryan assured Van Susteren that they’d figure out how to make Mexico pay, someday. “There are a lot of different ways of getting Mexico to contribute to doing this,” he said. “There are different ways of defining how exactly they pay for it.”

WATCH: Who’s going to pay for the border wall? Here’s what @SpeakerRyan just told me – more tonight on #ForTheRecord at 6pET on @MSNBC! pic.twitter.com/VCQgVM4Goj — Greta Van Susteren (@greta) January 25, 2017

During the campaign, Trump suggested that the wall could be paid for by holding hostage the funds that Mexicans living in the U.S. send home until Mexico turns over a “a one-time payment of $5 to 10 billion.” Some have questioned the legality of that plan, but other options have been floated, such as taxing goods entering the country from Mexico at a rate of 20 percent or increasing fees for visas and border crossings.

While Republicans are quickly forging ahead on construction of Trump’s wall, they haven’t elaborated on the strategy they’ll use to make Mexico finance it. Ryan’s remark that there are “different ways of defining” how Mexico pays suggests Americans may need to broaden their definition of what “reimbursement” means.

Trump echoed this theme in an interview with ABC News’ David Muir on Wednesday, insisting, “We will be in a form reimbursed by Mexico, which I’ve always said.

“We’ll be reimbursed at a later date from whatever transaction we make from Mexico,” Trump continued. “I’m just telling you there will be a payment. It will be in a form, perhaps a complicated form.”

In a nationally televised speech on Wednesday night, Mexican president Enrique Peña Nieto said, “Mexico does not believe in walls,” adding, “I have said time and time again, Mexico will not pay for any wall.”

But he may not have a choice in the matter. If the definition of “reimburse” is stretched to Mexico transferring money to the U.S. in a “complicated form” from “whatever transaction,” Trump can declare he’s made good on his promise to “make Mexico pay for it,” even without its consent.