$100 billion: That's roughly how much the U.S. Treasury Department says Iran stands to recover once sanctions are lifted under the new nuclear deal. The money comes from Iranian oil sales and has been piling up in some international banks over the past few years. But there are questions about what Iran will do with this windfall.

Oil is one of Iran's most valuable commodities. And, sanctions or no sanctions, Iran found buyers over the past few years. Month after month, millions of dollars of oil revenues were added to its ledgers. But Iran hasn't been able to get its hands on that cash. It's frozen in overseas banks.

"The money is sitting in China, India, Japan, South Korea, Turkey, maybe a little bit in Taiwan," says Mark Dubowitz, the executive director of the Foundation for Defense of Democracies. "Those countries were buyers of Iranian oil."

Dubowitz, a sanctions specialist who is critical of the deal, says Iran hasn't been able to access the roughly $100 billion sitting in those banks because of sanctions imposed by the U.S. in 2012.

He says the mostly Asian nations buying oil from Iran agreed to hold the funds in escrow until the sanctions are lifted. In other words, Iran sold them the oil but couldn't move the cash back home. However, it was allowed to spend the money to buy goods from those countries.

"That's why you saw Chinese goods, in particular, flooding Iranian stores and markets," Dubowitz says. "But the problem is, try as they could, they couldn't find enough to buy to spend down those accounts. So the money effectively continued piling up as Iran sold more oil."

Dubowitz says the system basically cut off Iran's access to the bulk of its foreign reserves. But with the nuclear deal, Iran will be able to access the $100 billion after the International Atomic Energy Agency verifies that it has implemented nuclear-related measures under the agreement.

These initial inspections are likely to take six months to a year. In the meantime, Dubowitz says there's concern about what Iran will do with the money once they get it.

"We have no ability to constrain Iran if they want to spend all $100 billion on funding Hezbollah or other terrorist organizations," he warns. "But when you're getting a $100 billion-plus cash windfall, even if you're spending 5 to 10 percent of that only on the regional activities and your support for terrorism, that's an extra $5 to $10 billion dollars-plus."

During a press conference on Wednesday, President Obama conceded that some of the money could be used to stir up trouble in the Middle East.

But Obama said Iran's shattered economy needs to be rebuilt after years of sanctions — and it is their money.

"We're not writing Iran a check," he said. "This is Iran's money that we were able to block from them having access to."

It's very likely the bulk of the $100 billion will not even be sent back to Iran, says Elizabeth Rosenberg, who worked on the Iran sanctions issue at the Treasury Department and is now with the Center for a New American Security.

"Something that Iran will be interested to do is get access to that money and move it to places where they'd like to invest or do deals," she says. "That may mean moving it into different currencies as well. And once they can move it into Europe, for example, they'll be able to engage in different purchases or investment opportunities and seek new partnerships."

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