I couldnt figure how the Bucs could possibly have a claim on Plummer's signing bonus. So bear with me while I try to explain what I think I understand.

Plummer and the Broncos renegotiated their contract in March 2005. This deal was transferred to the Bucs two years later in return for a conditional draft pick.
Plummer was benched in 2006 and told many people that he planned on retiring at season's end. The only problem with that was he had two years left on his contract and likely would have to repay a prorated portion of that signing bonus which he had already received.
On March, 2, 2007, Plummer faxed a letter to the NFL office stating his intention to retire. The next day, the Broncos traded his contractual rights to the Bucs in exchange for a fourth-round selection in the 2008 draft if he played, or a seventh-rounder if he didn't.
He did retire and the Broncos used the 7th round draft pick and the Bucs expect a portion of that signing bonus for the 7th round draft pick. The Bucs are settling for $3.5 million and this saves Plummer money because precedent is on the Bucs side.
Barry Sanders had to pay the Detroit Lions $5.5 million in 1998 when he retired early.
In 2004 the Dolphins won an $8.6 million judgment against Ricky Williams when his contract was traded from the New Orleans Saints.
It makes a little more sense to me now. I got the info from this link:
Plummer, Bucs near settlement - The Denver Post