I don't see a reason to release Slay after we play the Eagles on the 28th. We could see them again in the Super Bowl, however unlikely that may be. And we shouldn't give them any resources they can use to beat us at any point when we still might play them again. Can you imagine losing the Super Bowl to the Eagles on a Darius Slay interception because we decided "nah, he's not that important to Philadelphia and the chances we both make the Super Bowl are tiny, let's just be nice and let him go where he wants."
I'd be okay with releasing Slay and letting him go to the Eagles after it's assured there is zero chance we will see the Eagles again this year. If we don't qualify for the playoffs or are eliminated before the Eagles are.