The source said the offer sheet is a one-year, $2.5 million deal. The Steelers have five days -- until the end of Sunday -- to decide whether to match the offer or let Sanders go to the Patriots and receive a third-round draft choice from New England in return.
The Steelers are not expected to make a decision until Sunday.
The Patriots also Wednesday reached agreement on a one-year deal to re-sign wide receiver Julian Edelman, a league source confirmed to ESPNBoston.com.
The money in Sanders' offer sheet won't scare Pittsburgh, sources said. But Sanders can leave in a year as an unrestricted free agent. Instead of risking losing him with no compensation after the 2012 season, the Steelers could opt to take New England's third-round pick now.
Sanders, 26, is a quick, athletic receiver who caught a career-high 44 passes in 2012, doubling his production from the previous season.
He received an original-round tender from the Steelers only because of the team's salary-cap restraints this offseason.
Edelman, 26, played in nine games during the 2012 season, finishing with 21 catches for 235 yards and three touchdowns.
He finished the year on injured reserve after suffering a broken foot in a Week 13 matchup against the Dolphins.
The former college quarterback has been a valuable contributor for the Patriots in four seasons with the team, playing receiver, defensive back and serving as a core special teams player and punt returner.
Information from ESPNBoston.com's Mike Reiss and Field Yates, AFC North blogger Jamison Hensley and AFC East blogger James Walker was used in this report.