You should write him back and tell him that not only that you will do the 20,000 word translation for 600 Euros, but you will throw in a "good translation" for free. Get yourself on his contact list of translators. You never know when he might get back to you. Establish yourself as a quality, reliable, deadline, oriented resource.
Not going to work. Besides, I did tell him that I'd do it (at the very least) for a clean payment (no costs to me) of 600 euros. And I can guarantee you he won't write back.
To be honest, it would be a chore to do this for 600 anyway, but at least it wouldn't be insane like what he suggested.