Donald Trump proposed to block billions of dollars in payments immigrants send back to Mexico if the government there refuses to pay for a border wall, in a potentially devastating move for Mexico’s economy.

Trump, who has made building a wall between the U.S. and Mexico to curb illegal immigration a centerpiece of his Republican presidential campaign, told the Washington Post he would force Mexico to pay for the wall by threatening to stop remittances until the nation made “a one-time payment of $5-10 billion” to the U.S.

“It’s an easy decision for Mexico,” Trump said in the memo, which cites a section of the USA Patriot Act antiterrorism law that he argues can be changed to enforce his proposal if he is elected president.

It’s unclear if Trump would be able to carry out the plan without approval from Congress. Experts interviewed by the Post said that such an interpretation of the rights defined by the Patriot Act would be extremely expansive and likely subject to litigation.

Trump said the majority of remittances Mexico receives are from undocumented immigrants, though the U.S. Government Accountability Office said this year it’s difficult to measure that figure.

$25 Billion

World Bank data show Mexico gets about $25 billion in total remittances annually, and while that includes payments originating in other countries, the bulk is from the U.S. The data also show that remittances accounted for about 2 percent of Mexico’s gross domestic product in 2014.

Trump has said the U.S.-Mexico border wall would cost between $8 billion and $10 billion to build, and he has said if he is elected he would make the Mexican government pay the cost. Officials in Mexico have repeatedly said they would not fund the border wall.

Mexican President Enrique Pena Nieto’s administration said last year that Trump’s plan to bill Mexico for the wall “reflects an enormous ignorance for what Mexico represents, and also the irresponsibility of the candidate who’s saying it.”

A Trump campaign spokeswoman did not immediately respond to a request for comment.