The contract Justin Champagnie signed with the Celtics this past season had a team option, which is often put in so the team can potentially use the salary to help match up in trades. For the acquiring team, they can simply not pick up the option and they save cap wise. Of course the option was also there in case Champagnie impressed and the Celtics wanted to retain him for another season.
If Champagnie had made the opening night roster, his full salary of $400k would have remained on the books. The Celtics have two open roster spots now, but I'm not sure if they will use both. Blake Griffin potentially could end up taking up one of them if the team decides to bring him back.