The Oakland Raiders -- the busiest team in free agency -- became the first team this offseason to execute an offer sheet in restricted free agency.

The Packers have until Monday to match the Raiders' offer sheet to Sean Richardson. Tom Szczerbowski/Getty Images

The Raiders signed Green Bay Packers backup safety Sean Richardson to a restricted free-agent offer sheet on Tuesday. The Packers have until Monday to match the offer. Because Richardson was undrafted and received the low restricted free-agent tender of $1.542 million, the Raiders would not have to give Green Bay compensation if it declines to match.

It's a one-year, $2.55 million offer, a source with the Packers told ESPN.com.

Both the Raiders and the Packers are flush in salary-cap room at this point in free agency. Oakland, which has signed a total of 12 players this offseason, has $23 million in cap room, the fifth-most in the NFL. The Packers have $18 million, the seventh-highest in the league.

Richardson was undrafted in 2012 and was signed by Green Bay, months after Reggie McKenzie left the Packers' front office to become Oakland's general manager. Richardson has not started an NFL game. He is considered a valuable special-teams player and a key backup, especially against the run. The Raiders' starting satieties are 38-year-old Charles Woodson and Nate Allen, who signed a four-year, $23 million deal in March. Richardson would be a backup for Oakland, at least in 2015. He is a Vanderbilt product.

Richardson overcame neck fusion surgery after suffering an injury as a rookie in 2012. He missed the first half of the 2013 season but returned to play six games plus the playoffs.

The Packers' two starting safeties, Morgan Burnett and Ha Ha Clinton-Dix, are both returning, but Green Bay views Richardson as a quality backup.

Dix took to Twitter to express his hope that the Packers can keep Richardson.

Sean please don't leave me. — Ha Ha Clinton-Dix (@haha_cd6) April 7, 2015

Richardson's offer sheet came on the same day that Oakland safeties Larry Asante and Brandian Ross signed restricted free-agent tenders.

Restricted free-agency movement has become rare. From 2005 to '08, 15 players moved in restricted free agency. If Richardson goes to the Raiders, he would be the second since that time period. Receiver Andrew Hawkins went from the Cincinnati Bengals to the Cleveland Browns last year.

ESPN.com's Rob Demovsky contributed to this report.