Does he have an option on his final year? a couple of seasons ago in a world, I came back to an old team midseason for an abondened owner, it had actually passed hands 5 times since I left the first time around. Anyhoo, there were some familar names on the roster that we resigned by the owner immediatly following my 1st runs, both had mutual options at the end of the season for significantly more money than they were worth. They were both once occasional all star pitchers, I think both racked up 200 wins, etc... I waived them, and when nobody claimed them, I sent them down to low A towards the end of the season to help that team with a minor league playoff push. At the end of the season, both declined their options and retired.
I don't think a player will retire as long as they have a garunteed contract on hand for the following season, but going down to the minors triggered them to decline their option, then their age and rapidly declining skills sent them to retirement.
Other than that, you're pretty much stuck short of someone else picking up their deal.