Author here, and I agree with you. The phrase I hear most often is "I'm negotiable" or "but that's negotiable", which is why I used "I'm" for the title. I even

Author here, and I agree with you. The phrase I hear most often is "I'm negotiable" or "but that's negotiable", which is why I used "I'm" for the title. I even looked up the definition to see what was proper, and most definitions would imply that the term negotiable should apply to the request/demand and not to the person making it. So the amount requested is actually the noun that negotiable describes. If you Google "negotiable", the first thing that pops up was a definition (which seems pulled from a site called Oxforddictionaries.com) leading with "open to discussion or modification". "I'm willing to negotiate" does sound better, but might make for a worse title.