First, Plummer has only threatened to retire. He hasn't sent in his papers to the league office or the Broncos, yet. Basically, Jake was saying "I'm not reporting to Tampa Bay." And the Bucs pulled out of the deal.
If Plummer actually retires, the Broncos have a couple of options. One, they can release Plummer from his contract and ask for whatever pro-rated amount left from his signing bonus (that's what Jake probably wants). Two, keep Plummer's contract on the books, and treat him like a player who did not report. Essentially, it's a game of chicken. I would think the Broncos would release Plummer so they could utilize the cap space. That's probably Plummer and his agent's thinking, as well.