There are three ways to retirement:
1. Your contract ends and you sign no new contract. Your team would hold bird rights, this would be like if Scal signs with no one and hangs out as an assistant with Boston, then a trade comes up and we need to match salaries, so we resign Scal on a one year + player option to make the deal work.
2. You retire and your team lets you retire, but they keep paying you. In this case you keep getting paid, but still count on the cap.
3. You retire and the team forces you to file paperwork with the league. In this case you are cleared from the books.
(Not looking at injury piece.)
Another option would be to buy him out. I'm pretty sure he could agree to a buyout in advance of a trade, so the receiving team KNOWS he won't renege. The C's could send the cash for the buyout.