Secretary of State John Kerry is headed to Vienna for more nuclear negotiations with Iran. The deadline for a deal is June 30. 

Kerry has lots of tools at his disposal as he works with U.S. allies to convince Iran to curb its nuclear program. The sharpest tool is sanctions, which have taken a huge bite out of Iran’s oil exports. Iran still exports some oil to a handful of countries, but oil payments can’t go through Western banks. 

Gary Sick was on the National Security Council under presidents Ford, Carter and Reagan, and is now a senior research scholar and adjunct professor at Columbia University’s School of International and Public Affairs. He estimates Iran is owed around $40 billion.

“It’s their money,” he says. “They were paid that money to sell oil to these countries, but they can’t get their money.” 

Sick says Iran is definitely feeling the pinch, because the U.S. and allies in Europe, as well as China and Russia, support the sanctions. But that support could wither if Iran found a way to avoid Western banks, say some analysts.

“Presumably, the Iranians would sell their oil at a discount over the global price,” says Jon Alterman, a senior vice president with the Center for Strategic and International Studies. “And if you could save $6 a barrel buying Iranian oil, there are people who’d say there’s a lot of money in that.”