Ainge is holding out for Bradley, who he probably won't re-sign. But Crowder makes more sense for both teams, because Bulls have Wade at the 2, and Butler would be more suited for the 3, replacing Crowder. Butler doesn't shoot the 3 much, so he wouldn't be as desirable at the 2. And they still have Brown as the backup 3.
I don't see this as a deal blocker, if there really is a deal. Ainge has a lot to gain from the savings of Butler's contract--with Crowder or Bradley. And if the deal includes Nets 18, not 17, it's a no brainer.