Didn't the Raiders initially use their franchise tag or threaten to use it on Seymour after the 2010 season? That might have factored in his decision to re-up with them. Leopards don't change their spots, but human beings mature (hopefully) and change priorities sometimes.
You can even make an argument that if it's solely about the greenbacks, then returning to the Pats might be his best option. At his age, he's unlikely to get long-term, megabucks contracts going forward. Returning here would improve his legacy especially if the Pats should return to SB glory with his return. In football, legacy can be turned into serious dollars once your playing career is over. Just ask all those Pats alumni drawing handsome checks from their post-career media gigs.