They could take his salary and break it down into 15 million this season and 15 million next season but guarantee it while having a voided contract. That would give Carr his entire salary while giving cap relief, and still make it more attractive for a team to trade for him as he would have a reduced cap hit this season for receiving team (15 million), and only carry a 15 million dead cap hit next season or they could rework his deal if they keep him.
It would be a win win for Saints and Carr IMO, while dropping his 61 million cap hit next season to somewhere around 38 million (all dead money), and he becomes a FA next offseason. Not ideal scenario, but not the worst.