I would not be opposed to trading for Fitzpatrick, but I don't know that I would use that as reasoning for trading Peterson.
If you told me that I could have Peterson, Murphy (on a rookie deal), and Fitzpatrick (on a rookie deal) as my CBs for the 2020 season, and using that depth to negotiate an extension for Peterson...I would do that. 3 CBs are almost a base defense at this point in time and IMO, Peterson will have a Charles Woodson late career with a move to S.
I think with the Dolphins tanking, you could make an argument for Alford and a 3rd for Fitzpatrick. Just that way they have a no-bonus CB to fill out their roster who they can cut at any time with no penalty.