Campbell doesn't seem like a paycut candidate, but I do think he's an extension candidate to flatten his cap hit a bit.
He's due $10M salary ($5M of which is already guaranteed so releasing him doesn't make sense). Could add a year to his contract that converts salary to bonus and spread it into next year. This could be a purely on-paper change, the contract could be written so it automatically voids after the year. Convert that $10M salary to $2M salary and $8M bonus and then instead of counting $10M in 2021 it'd be $6M in 2021 and $4M in 2022. You would lose any comp pick benefits that Campbell would have as a FA if his deal ended after 2021, since you'd technically be cutting him after the season.
Alternatively if they actually want to set up for him to return in 2022, you just agree on a salary, probably have to guarantee a small amount of it so that Campbell has reason to lock himself into a number.