Yea, this has happened a few times before. I don't know how it works contractually, but its a mutual decision.
The only reason it really makes sense for Acy is if the team is telling him they are GOING to keep him. The longer free agency plays out, the less likely a roster spot will be open for him.
I view this as a good sign a trade will happen before that deadline, and Acy will be back. Both good things.