The last thing you want is to get blindsided by a fast-talker. Email is great because it slows down the process and gives you time to think. Also, email gives you a written record of the conversation. If they try to pin you down on the phone, you can say, “I’m happy to get you that information. What’s your email address so I can forward you a proposal?”
Source: Nine Biggest Negotiation Mysteries Solved | Psychology Today
Negotiating via email is tricky. Here’s the skinny.