Come on now...
A struggling winger (that has asked to be traded) + a B-prospect + cap dump.
Carter should get at least a prospect + 2nd/3rd (depending on the prospect). Add in the cap dump in Benning and the plus that would be attached to Carter and that should definitely be enough.
If you go by what people in the industry have said, there's little interest in Carter. Until shown otherwise, it's best to assume that no potential will come back to the Kings for him. Edmonton might take him, if the Kings took back 2 or 3 cap dumps. Take a couple guys the Oilers don't want, their contracts end after next year, trade them at the deadline or let them walk, either way. That's clearing his contract, which is most likely the only thing the Kings can do at this point.
If he comes back next year, and looks like he'd rather be on the ice than at home, and is truly a productive player, then maybe the Kings can get some potential in a trade. Nobody needs a 13g, and 33pt Carter who will be 35 next year, with 2 years left after next season. He may even choose to play those 2 years, despite any back diving of his contract. His buyout after next year isn't really a good one either.
There is only risk for anyone trading for him. There is some potential for reward, but only if his legs are good, which at 34/35 years old is no longer a guarantee.