Well this is an interesting question and I'm not going to answer it directly. Obviously you've proposed it in such a way that the answer is "you can't keep him", although I don't think this really reflects how it would be done.
I'd look at the market, and see what is available. If any gun player is willing to come to Adelaide for $850k, seeya Jake! If we're going to get a great draft pick, seeya Jake! And **** off too, while you're at it.
But if there's no one available, all our trade targets fall through, and/or Jake's destination club isn't offering us enough at the draft table, I'd retain him on the salary he wants. Because if there's no better option, then the best option becomes retaining him.
You look at the Tippett, Gunston, Dangerfield deals and so forth. What did we do? Well Tippett gave us draft sanctions, Gunston gave us literally jack shit, and Dangerfield we got some pretty mediocre picks. We didn't do well out of those players leaving. Imagine if we got some seriously good players in return; I doubt we'd be complaining as much.
Would I rather trade Lever for some junk late first round pick, or retain him for $200k p/a more than he's worth? I'll take the latter every single day of the week thanks.