Pressure is increasing in Congress to impose more stringent sanctions on Iran, but the actual target of congressional ire and the ire of Israeli and Arab leaders is Secretary of State John Kerry, who seems to many critics to be starry-eyed and frenetic in his approach to just about every Middle East crisis.
When the foreign minister of France suggests that you are in danger of accepting a "fool's deal," perhaps you are in danger of accepting a fool's deal.
Sanctions, combined with the threat of Israeli (and to a lesser extent, American) air strikes, are what brought the Iranians to the negotiating table. So it makes a certain amount of sense to argue that even heavier sanctions will concentrate the attention of the regime that much more.
But the advocates of more stringent sanctions forget one thing: Their audience is not merely a cadre of mullahs in Tehran. Many countries support U.S.-led sanctions. Not all of them do so with equal levels of enthusiasm. If these countries, which include China and India, begin to suspect that the United States is overly intransigent at the early stages of these negotiations, they will be more likely to lose faith in the U.S.'s good intentions and move to undercut the sanctions regime.
If China and India (and South Korea and Japan) decide that the U.S. is uninterested in reaching a deal, this will actually make the American task of keeping the sanctions in place much more difficult. And Iran will be in a better position to wait out international opposition to its nuclear activities.
On the other hand, easing sanctions, as many members of the P5 + 1 (the permanent members of the United Nations Security Council plus Germany) seem happy to do, makes little sense unless the Iranians agree to suspend their uranium-enrichment activities and halt the building of the plutonium reactor at Arak for the duration of these talks (talks that to be successful must conclude with a guarantee that Iran won't be able to manufacture nuclear weapons).
The Iranians have a history of expanding their nuclear program under the cover of negotiations; the least Western diplomats could do to avoid looking like suckers is to demand that Iran press the pause button. At that point, loosening Iran's access to its now-frozen foreign currency reserves makes sense.
Western negotiators will only demand enrichment suspension if they remember that they have the advantage here: Sanctions are crippling Iran's economy, not the West's.