The announcement's timing, just after confirmation that three Americans left Iranian airspace, is leading to calls for investigations and shedding light on a little-known fund that the president can dip into when he wants to resolve international financial disputes. Legislative efforts are already afoot to curtail that ability.
US officials deny claims that the payment was a bribe to ensure the release of a total of five Americans traded for the freedom of seven people in legal trouble in the US over business deals with Iran.
With the United Nations' confirmation that Iran satisfied the terms of last summer's nuclear agreement, it immediately recouped tens of billions in frozen assets and earned the chance to gain significantly more from suspended oil, trade and financial sanctions.
The much smaller US-Iranian agreement concerned more than USD 400 million in Iranian money, dating back to before the 1979 Islamic Revolution and the end of diplomatic ties, which the US-backed shah's government used to buy American military equipment. The Iranians got that money back last weekend and some USD 1.3 billion in interest.
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Earlier this week, however, one Iranian military commander painted the payment in a different light. Mohammad Reza Naghdi, head of the Basij paramilitary wing of the powerful Revolutionary Guards, said the wiring of the funds was a payoff for letting the Americans go.
US officials insist that's not true.
"There was no bribe, there was no ransom, there was nothing paid to secure the return of these Americans who were, by the way, not spies," State Department spokesman Mark Toner responded, referring to the charges that held each of the Americans in Iranian prison for years.