Greetings Mike:
I hate to sound like a broken record, but I'm sure you are aware that it is nearly impossible for me to tell you whether or not you have received a fair price for said tree. Obviously, I cannot physically see your tree; thus, I cannot conclude with any certainty to the measurements you've described are indeed accurate to quality and quantity of said tree.
Additionally, I have several questions that were not addressed in your request: Is this a yard tree? Which species of walnut (Juglans nigra; J. regia; J. hindsdii; J. californica; etc.)? Why are you removing this tree? Is the tree physically located in the state of Washington? etc.
If this is a run of the mill yard tree and you're going to receive free removal PLUS $2000, I say get it in writing and sell the darn tree! Again, I can only go on the information you provided me and that information at best was very limited. Please understand that I cannot give you an appraisal because appraisals must be conducted on-site (no exceptions).
By the way, most homeowners I know of who try to sell black walnut yard trees end up paying a company to remove the tree simply because there are very few individuals who are willing to dabble in the risky business of removing and buying urban trees for traditional milling. Granted, many companies will take the logs or pay you a "little" for the logs after someone else cuts down the tree. HOWEVER, there are custom sawmill operators out there who love urban trees and who specialize in milling urban trees--all you need to do is find these custom sawmill operators!
Now, if you absolutely want to make sure you are not getting hosed, please contact your local extension forestry or state forestry office in WA!
Best of luck!