Terms were not immediately available Friday afternoon.
McGee, Buffalo's top cornerback the past two seasons, missed Friday's practice because of back spasms and is questionable for Sunday's game against Tampa Bay.
The 28-year-old led the Bills with three interceptions last season and is the team's all-time leader in kickoff returns (206) and kickoff return yards (5,420).
The seven-year veteran has 16 interceptions in 90 career games, and earned Pro Bowl honors in 2005. He was also a Pro Bowl alternate in 2006.
He was playing in the final year of a four-year extension signed in 2005.
McGee developed the spasms Friday morning after practicing all this week. If McGee can't play on Sunday, Ashton Youboty likely would take his place.
Starting left defensive end Chris Kelsay practiced Friday for the second straight day and should be ready for the Bills' home opener. Kelsay bruised his knee in Buffalo's season-opening loss to New England, and missed Wednesday's practice.
Starting right defensive end Aaron Schobel returned Friday after sitting out Thursday's practice because of a sore back, and is also probable for Sunday's game.
Information from The Associated Press was used in this report.