Brian's a great reference here. I'm just not personally sold that they're required to forfeit a prorated portion of his their bonus. Maybe they are, or maybe he's just assuming the Ravens will ask him to. I've seen numerous articles and reports, all the way back to guys like Calvin Johnson and Andrew Luck retiring, that say different things about what teams can and can't do in those regards. Ultimately it sounds like it ends up as a grievance filing that goes to arbitration, if that's the route they go.
My "assumption" is that if the Ravens are interested in keeping him on the 53, you wouldn't go after the bonus part. Maybe if you cut him you do, but it seems like more of a "nice guy" gesture to let him keep the $300K or whatever it comes out to.