Does the president need approval by Congress for lifting the sanctions? I don't know the answer, but I suppose that might be an issue.
Congress must pass a bill to revoke the US sanctions, which is a separate act of legislation from assenting to this deal. There are several Executive Orders which Obama can revoke but the embargo proper is embedded in US legislation.
[1]: http://www.nytimes.com/2015/07/15/world/middleeast/congress-...