I don't think Charlotte is going to up their offer for Kemba. They have no salary flexibility if he signs to stay under the tax and they absolutely will not pay the tax. If they won't pay the tax they cannot add pieces to the team.
They currently have 97 million in salary, and that's without Kemba. Add at his max of 32 Kemba and they are essentially at the tax line as is, with Jeremy Lamb their second leading scorer also a FA.
If they wanted to keep Kemba they probably would have been trying to dump salary during the draft, its been reported they weren't doing so.