The $1.8 (actually $1.7 billion) was a debt owed to Iran, which bought military equipment from the U.S. that it never received because relations ruptured when the shah was overthrown in 1979.
The debt was in international arbitration for years. As part of that, Iran paid settlements of more than $2.5 billion to U.S. citizens and businesses.
$400 million, representing the principal and held in a U.S. government trust fund, was paid in cash and flown to Tehran on a cargo plane, which gave rise to Trump’s dramatic accounts of money stuffed in barrels or boxes and delivered in the dead of night.
The remaining $1.3 billion, representing interest accrued over nearly 40 years, was paid separately. In order not to violate U.S. regulations barring direct U.S. dollar transfers to Iranian banks, the money was remitted to Iran in late January and early February 2016 in foreign hard currency from the central banks of the Netherlands and of Switzerland, according to the Congressional Research Service .