At this point the sanctions are gone even if Congress rejects the deal. A Congressional rejection will mean there’s no deal with the U.S. AND there’s no more international sanctions on Iran. The United Nations Security Council already voted to end international sanctions. Russia (and China) will veto any attempts to reimpose international sanctions. The U.S. rejecting the deal will not stop international sanctions from ending and soon Iran will be trading with Europe, Asia and Russia the goods that were previously sanctioned, irregardless of what Congress or the U.S. does.
Agreed with OP NeutiquamErro.