If Wilson is cleared he has a right to return to work.
Unforunately, sadly, the Ferguson police have strong incentives to avoid that.
Which is why there are negotiations, and why Wilson actually has the upper hand in them. It wouldn’t surprise me at all that an immediate vesting of full pension (plus health benefits) based on a projected 20 years of service (rather than his actual 2) was part of the deal. Or perhaps a lump-sum payout based on estimated lost full career wages.
I doubt he'd get anything close to that. Rather than bite a financial bullet like that the Ferguson PD could easily cook up some procedural mistake Wilson made during the Brown shooting and dismiss him for cause. He'll remain on the force until it's clear he's off the hook in terms of any changes and then he'll resign.