WOW, I'm impressed that there is a VW salesman that knows his product and also cares to give the best trade value possible.
I too sell VWs, but only used ones. As you already know, it is very difficult to fix a price on an older car with abnormally low miles. Not what we usually see on a diesel. I think he could get his best price by selling it himself. I know it is easier to let the dealer handle everything, but this is a special case.
Also, I think the word is out that the 04 and 05 Passat are to be avoided. I happen to disagree, but then again, my 05 has not sold yet. Actually nothing is selling of late.
Good luck with this. Sorry I could not fix a price for you. If you PM me, I'll tell you what I paid for my two TDIs.