The major problem with the Iran nuclear deal is that the US had sanctioned billions of dollars in assets in bank in South Korea and Japan that the Iranians had earmarked for their nuclear program. When Obama and Clinton bartered this deal, they opened up the banks and Iran has been transferring billions into Germany and the United Emirates for their use to go back to work on their programs. This will put the Iranians right back on track to have a nuclear device well ahead of time. Thanks Obama.
http://www.latimes.com/world/middleeast/la-fg-iran-frozen-assets-20160120-story.html
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Actually, the biggest issue might be where Obama came up with $400 million to pay for US citizens held hostage by Iran.
Congress did NOT approve any such thing.