Why do you think ZB is trying to "bargain" for a different role on the Bills? It seems to me he was told what his role would be, he said "no", and he moved on. Not any different than a player preferring to sign with a team where he has the opportunity to start.
As for the money he was offered by the Bills, we have no clue. All we know is he fired his agent and hired a new one. If an offer was still on the table, his new agent would be a fool not to have his client sit down with the Bills at least once more.