When the Tampa Bay Buccaneers released Carl Nicks, they did so after reaching a settlement with him. He was owed $6 million in guaranteed money, and he had no reason to give up any of that money. After releasing Nicks, the Bucs are now $16.7 million under the salary cap, per the NFLPA's record-keeping (h/t @FoedusFDG). Prior to his release, they were $10.3 million under the cap.
This implies that the injury settlement consisted of Carl Nicks giving up all or nearly all of his $6 million guaranteed salary. If that seems unlikely, that's because it is: according to Jason of Over The Cap, no injury settlement has been incorporated yet. But keep an eye on that salary cap page: as soon as you see that number dip again, you'll know how much Nicks gave up in his semi-retirement.
One interesting possibility was raised by Joe Bussell: the Bucs could have converted some of that salary into a bonus to pro-rate the cap hit. This would not be particularly useful given the fact that all salary cap space rolls over into subsequent years anyway, but it's an option. Regardless of the settlement, pro-rated dead money will hit next year's cap at $4.7 million as a result of the late 2012 conversion of salary into a pro-rated signing bonus.