Corey had a $3M roster bonus due in 2006. After being ineffective due apparently to injury (BB said he always worked hard) he was asked to defer that to 2007. In 2006 he was still obviously hampered by something - injury, age, loss of killer instinct, whatever. So the team wasn't going to ever pay his deferred roster bonus. Corey knew this at the end of the season. Maybe he could have stayed at a reduced salary in 2007, but it would have had to be reduced dramatically to lower his cap hit. Players will sometimes play for less somewhere else, seldom for their present team because they tend to feel that there they have "earned" an overpayment or two. Ergo, he won't be playing here again.
I doubt he will play anywhere because I think Corey is done mentally and physically. He could push through some of that, but it would likely take a stupid overpayment to entice him to undertake the effort to try to. His agent is probably still hoping someone will make that offer, since 3% of Corey retired = $0. It would probably take $3M plus big sounding makeable if he's starting as a featured RB incentives to get Corey off his couch.
As for that stat that claimed we got more bang for the buck from Corey than Maroney, that's nonsense. Sometimes you have to trust what your eyes see (as opposed to crunching numbers in a vacuum). Corey had a $2.6M cap hit in 2006, Maroney's was $1.1M. He got no signing bonus (although his deal is highly guaranteed $6.1M of an $8M+ total but that is spread over 4-5 bargain seasons for an explosive young RB). Corey may have scored more TD's, but that was a function of his short yardage work coupled with a sub par WR corps and an injured rookie for the last third of the season. And a third more TD's for more than twice the compensation doesn't equal way more bang in my book. When you look at total yardage, they were in the same ballpark with the edge on number of longer runs/receptions going to the youngster and the edge on shorter scoring runs going to the veteran. Take out Corey's lone 50 yard run of the season and that becomes more apparent. He could still be a bull in spurts, but he could suddenly barely outrun my elderly mother.
If someone wants to pay him $3-5M to be a third down and goal line back - and even than WHEN HE's NOT WINDED OR DINGED - they will probably be tossing him under the bus come December because they were paying for and expecting at least a semi-featured back. Corey is a proud man and I think on some level he absolutely knows that. Irrespective of what his agent would like him and all the rest of us to believe.