Atlanta has a lot of cap room. Currently they're under the salary floor even. So while they could send back Carroll if they wanted to, there's no need.
It'd be pretty much a tank move, but what of Jeff Green just for Atlanta's 2015 pick, unprotected, which will be the better of their or Brooklyn's? Brooklyn was a bit shaky last year as is, and with another new coach, the loss of Pierce and Livingston, no significant additions unless you're a Jarrett Jack fan, the improvement of most of the top teams in the East, and the major improvement of the Cavaliers, that pick could very easily fall into the lottery. Would you do it? Would Atlanta do it? If you don't think Atlanta would do it, what if we offered them a pick swap -- the least valuable of our pick and the Clippers pick for their Hawks/Nets pick? Again, it's a total gamble, but Brooklyn looks set to plummet this season.
As I've said in many threads, I do think the Hawks are an especially good fit for Green. He'd be a big upgrade for their 3, and combine that with Horford coming back and you have a team that's ready to make some noise in a conference lacking a truly dominant team.