Selling any player to a rival, who can cause harm to you latter is never great, but if the money is right, then it's worth to take the risk. I don't know if 12M justifies that risk. I'd rather sell him somewhere else and lose a 1-2M if that means weaker Napoli. Also, it may be worth to wait until the end of transfer window and wait for Zenit's offer if they indeed are in for him. They tend to go full retard in the last days, we could get a really good deal out of them.