They don’t have to wait to do that. You can cut a guy with a post-June 1 designation before June 1. He stays on your salary cap until June 2, but the deed is done and the player is free to sign with another team. But they save another $2 million against the cap if they trade him post-June 1, rather than cutting him.Not sure I follow the logic. They are waiting for the same reason Jones is a post June first. To spread the dead cap over 2 years. Once they make the cut, they no longer have his rights, and can't trade him to anyone. That's not to say they have to cut him immediately, but why would Miami trade for him and assume the contract, when they can just wait and get him cheaper as a FA?
Your post is a rationalization.
Miami would trade for him in this scenario because it guarantees them the player. If they wait for him to hit the open market, they risk the chance that someone else will trade for him before then, that he and Minnesota will work something out, or that Minnesota will hold onto him until way later in the off-season than we like. Then, even if he does hit the open market, Miami has to be the high bidder.
If they can deal a late round pick to avoid all of that and lock up the player, while the player agrees to a reasonable reworked contract that is acceptable to the Dolphins, it is the best outcome for all involved. Minnesota saves money and gets a late pick, Miami gets the player they want at the price they want, and the player goes to the team he wants and gets a reworked deal he likes.