^^^According to what I just saw on the NBC network news, the US did not pay anything from US assets. The US released $ 6 billion of oil revenue paid by South Korea to Iran for oil. That $ 6 billion was tied up in US sanctions against Iran. So it is not like the US wrote a $ 6 billion check to Iran.
https://www.wsj.com/articles/iran-releases-four-americans-from-evin-prison-b396ee09From the article
Iran has freed five Americans from a notorious Tehran prison with the understanding that five of its own prisoners and billions of dollars in frozen revenue from energy sales would be released in return, U.S. and Iranian officials said.
The official Islamic Republic News Agency said more than $10 billion would be released from accounts in South Korea and Iraq, and that under the agreement, the Americans wouldn’t be permitted to leave Iran until the funds become available in Iranian accounts in Qatar. A U.S. official dismissed Iran’s claim, saying that only the $6 billion in funds held in South Korea was under discussion. The official added that in exchange for having these Americans move to house arrest, no Iranian prisoners in the U.S. have been released.
“Iran will not be receiving any sanctions relief, and in any instance where we would engage in such efforts to bring Americans home from Iran, Iran’s own funds would be used and transferred to restricted accounts such that the monies can only be used for humanitarian purposes…permitted under our sanctions,” Secretary of State Antony Blinken told reporters on Thursday. “We will continue to enforce all of our sanctions. We will continue to push back on Iran’s destabilizing activities in the region and beyond,” he added.