WASHINGTON — A $400 million cash delivery to Iran to repay a decades-old arbitration claim may be unprecedented in recent U.S. history, according to legal experts and diplomatic historians, raising further questions about a payment timed to help free four American prisoners in Iran.

The money was sent to Iran on Jan. 17, the same day Tehran agreed to release the prisoners. The Obama administration claimed for months the events were separate, but recently acknowledged the cash was used as leverage until the Americans were allowed to leave Iran. Only then, did the U.S. allow a plane with euros, Swiss francs and other foreign currency loaded on pallets to take off in the other direction for Tehran.

“There’s actually not anything particularly unusual about the mechanism for this transaction,” White House press secretary Josh Earnest said. But diplomatic historians and lawyers with expertise in international arbitration struggled to find any similar examples.

Asked to recall a similar payment of the U.S. using cash or hard money to settle an international dispute, the office of the State Department historian couldn’t give an example.

The acknowledgment that the prisoners and the payment were linked, and the unusual cash delivery, have fueled Republican claims that a “ransom” was paid. At a news conference this month, President Barack Obama said cash was used because the U.S. and Iran don’t have a banking relationship after years of U.S. sanctions on Iran.

The $400 million was the principal owed by the U.S. on a 1970s Iranian account for buying U.S. military equipment. After Iran’s 1979 overthrow of the U.S.-backed shah and the U.S. Embassy hostage crisis in Tehran, the weapons were never delivered.