A great way to say 'no' is 'I'm sorry, but I'm afraid I just can't do that.'
Another great way to say 'no' is 'How am I supposed to do that?' These need to be said with a respectful, maybe even apologetic tone of voice. You want to be both collaborative and be able to set limits at the same time.
Trying to get what you want by cornering or dominating the other side is a recipe for them not following through because they want to cause you problems because they resent the deal.
Carlin Flora, a writer for Psychology Todayemphasized the importance of empathy: by being sensitive to what the other party wants to gain from your negotiation, you can be much more persuasive, and Voss’s approach reflects that:
My approach is trying to make the best deal possible, yes for myself but also so the other side wants to continue to do business with me. The definition of a high value trade is for both sides to get a lot of value they couldn't otherwise get.