The third side in a dispute

Never say "no" if there can be an alternative. Conditional responses. Counter offers. Affirmative defenses. Even a dealbreaker or a hard pass can be up for discussion. Nothing is set in stone. 

Your answer can be time-bound. Maybe it's a "no" for today. Your answer can be clarified. Maybe it's a "no" until you understand the consequences. Your answer can be qualified. Maybe it's a "no" if there will be other parties involved. Your answer can be challenged. Maybe it's a "no" until you find a middle ground.

If parties are only allowed to say "no," then they either walk away with nothing or stay, hold the line, and reach an impasse. In any case, no one wins.