Most likely it's like the Suarez situation. De Gea knows he's going to be at United for a little while, so he signs an extension that pays him more money while he's there, probably includes a sizable buyout clause, and RM activates it next summer. Everyone wins.
It's going to be a battle of which manager actually makes the obviously correct decision. Liverpool will need to start the match with a midfield 3 of Can, Milner, and Henderson. Somehow I'm not sure he does that though.