Petry is going to move in the summer for an offer that would get you mocked in this thread right now. I realize he's (AGE) and makes (DOLLAR FIGURE) for (NUMBER OF YEARS) but I don't think it's going to be particularly difficult to convince just one GM that the Petry they saw in July 2021 did not completely disappear in October 2021. It would be one thing if there was visible decline in his skating or his skills but that isn't the case, the tools are all still there - it's just a complete nightmare season for the Habs and there's a total of two players playing at or above expectation (Lehkonen, Wideman).
Duncan Keith moved for real assets in the offseason. Rasmus Ristolainen went for a first. The idea that there's not going to be a single team out there that has a hole at RHD and is interested in Jeff Petry at 6.25M if they can ship out a bad 3-5M contract at the same time is something I'm extremely skeptical of. All it takes is one team that has a hole at RHD and can be talked in to the family stuff/covid/short offseason as a bad situation that snowballed and he'll be back to normal with a fresh start.
The retention stuff seems to be approaching it from the wrong angle too, another team will want him because they think he'll fix their problems at 1RD or 2RD, at which point 6.25M is a perfectly reasonable number, particularly when you're shipping out a bad contract at the same time. If you don't think he can play in your top 4 anymore you don't really want him at 4.5M anyway, so I don't think it's going to be significant retain unless it's a team that doesn't have any bad contracts to send at all.