Since the earliest moments of his campaign for the White House, President Donald Trump has promised that he will build a wall on the Mexican border and that he will force the Mexican government to pay for it.

The promise to wring funds out of Mexico for a barrier meant to block its own people from entering the U.S. always seemed a bit far-fetched, and only became more so when the Mexican government angrily announced that it would never pay for Trump’s pet project. But Trump never stopped insisting that the funds for the wall would come from Mexico, even if he eventually began tweaking his explanation of how it would happen.

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The payment might not be a direct exchange of funds, he began suggesting over the past year, but might involve the U.S. extracting money from the Mexican government by other means. Later, he moved from the “will pay for” the wall to a “will reimburse us for the wall” argument. That change, he said, was so that construction could start right away.

In an interview with ABC News Monday, Trump said that he expects construction on the wall to begin within months and gave an even more convoluted explanation of how Mexico will “100 percent” be made to finance it.

“All it is, is we'll be reimbursed at a later date from whatever transaction we make from Mexico,” he told reporter David Muir. “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.”

On Tuesday afternoon, though, when he signed what the White House described as an executive order that would launch construction of the wall as well as significantly toughen border enforcement, Trump may also have dropped a more substantive hint about how he might be able to claim that Mexico “paid” for the project.

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Buried in the order is this direction to federal agencies:

“The head of each executive department and agency shall identify and quantify all sources of direct and indirect Federal aid or assistance to the Government of Mexico on an annual basis over the past five years, including all bilateral and multilateral development aid, economic assistance, humanitarian aid, and military aid.”

It goes on to submit that list to the secretary of state within 30 days. The order then gives the State Department another 60 days to “submit to the President a consolidated report reflecting the levels of such aid and assistance that has been provided annually, over each of the past five years.”

Mexico is one of the larger but by no means the largest recipients of foreign aid from the United States. The Mexican government and military received nearly $420 million in assistance in 2013, with most of the money going to economic development.

It’s unclear from the text of the order what the purpose of the data collection is, but it is at least plausible that Trump is considering one of two possible courses of action. First, he could present the figures to Mexico and use the threat of withdrawal of the aid as leverage to compel some sort of payment for the wall.

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Alternatively, he could cut out the middleman and just try to withhold aid payments until he feels he can make a credible case that Mexico has actually “paid” for the wall.

Whatever the eventual plan, the move on Tuesday is certain to anger the United States’ southern neighbor. Mexican President Enrique Pena Nieto is supposed to meet with Trump next week, but reports from the Mexican capital early Tuesday suggested that if Trump went forward with the executive order, he might cancel his trip to Washington.

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