I'm sure if they traded Peppers, they could come to an agreement with the team they are trading with of picking up some of the money.
I'm not sure how the tag works, but I'm fairly certain the new team either picks up half or all of the salary, or it's just the whole tag goes away and then they re-do a long term deal.
Plus, even if the Panthers had to pick up all of the contract, I'm sure there are guys who they can cut to work it out.