representing the principal from a decades old unfilled U.S. arms sale during the Carter administration.

The remaining $1.3B settlement of interest on the $400M was paid later.

The rest of the trope includes a mythical $150B payout. There was no $150B payout from the U.S. treasury. The $150B "transfer" refers to unfrozen Iranian assets held abroad that were frozen until the deal was reached, allowing Tehran to finally access its funds.

From the link below:

WASHINGTON (AP) — President Donald Trump likes to tell a story about the U.S. paying out billions of dollars to Iran as part of the multinational deal freezing its nuclear program and easing sanctions against it. What he doesn’t say is that most of that money was Iran’s to begin with. The rest relates to an old debt the U.S. had with Iran.

The numbers and some details change in his retelling — dating back to the 2016 campaign — but his bottom line is always the same: The Obama administration was hoodwinked into giving Iran all that money, some of it in a huge and hidden bundle of cash.

The latest iteration of his claim Tuesday and the reality behind it:

TRUMP: “The Iran deal is a terrible deal. We paid $150 billion. We gave $1.8 billion in cash. That’s actual cash, barrels of cash. It’s insane. It’s ridiculous. It should have never been made. But we will be talking about it.” — remarks before a meeting with French President Emmanuel Macron. At a news conference Tuesday, he spoke about “giving them, Iran, $150 billion at one point.”

THE FACTS: There was no $150 billion payout from the U.S. treasury. The money he refers to represents Iranian assets held abroad that were frozen until the deal was reached and Tehran was allowed to access its funds.

The payout of about $1.8 billion is a separate matter. That dates to the 1970s, when Iran paid the U.S. $400 million for military equipment that was never delivered because the government was overthrown and diplomatic relations ruptured.

That left people, businesses and governments in each country indebted to partners in the other, and these complex claims took decades to sort out in tribunals and arbitration. For its part, Iran paid settlements of more than $2.5 billion to U.S. citizens and businesses.

The day after the nuclear deal was implemented, the U.S. and Iran announced they had settled the claim over the 1970s military equipment order, with the U.S. agreeing to pay the $400 million principal along with about $1.3 billion in interest. The $400 million 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 arrangement provided for the interest to be paid later, not crammed into containers.