Antonyms for the phrase "clinches the deal" could include being unsuccessful in negotiations, losing an opportunity, failing to close the deal, or having someone else seal the deal instead. Other antonyms might be hesitating, procrastinating, or being indecisive, which would result in missing the chance to finalize a deal. To hinder the completion of a transaction, you could undermine trust or goodwill with the other party, or alter the terms of the agreement. Alternatively, you could withdraw from negotiations or refuse to compromise. All these actions would serve as antonyms for the positive phrase "clinches the deal.