Ask what/how questions instead of why.
Online Course: MasterClass | Chris Voss Teaches the Art of Negotiation
Why-questions tend to trigger defensive reactions in negotiation. Think about how you felt when some asked you "Why did you do that". By changing why to what, you can remove the sting of accusation.
For example, instead of asking "Why do you need delivery in three weeks on the product", you can ask "What makes it necessary to get the product delivered in three weeks".