Abbas Araghchi's comments come as US Secretary of State John Kerry and members of his nuclear negotiating team are being grilled in Congress two weeks into a 60-day review of the deal in Washington.
A similar process is under way in Iran's parliament, where some MPs have argued that the agreement has jeopardised the country's nuclear programme.
The deal between Iran and six world powers led by the United States must be approved by Iran's top security committee and parliament before it can be finally implemented.
"In that case we will not suffer any losses and we can return to our normal programme," he told Iranian state television's political editors.
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"Then the world would say that Iran went through its legal process and approved the agreement yet the Congress destroyed the whole thing."
Kerry has said it would be embarrassing to him and a blow to American credibility on the world stage if the Republican-led Congress rejects the deal to put an atomic bomb out of Iran's reach.
President Barack Obama can veto Congress and keep the deal alive as long as US lawmakers do not obtain a two thirds majority vote.
Araghchi, effectively the number two negotiator for Iran in the nuclear talks behind Foreign Minister Mohammad Javad Zarif, spoke ahead of President Hassan Rouhani's live televised speech on Sunday night which is expected to focus on the nuclear deal.