Feige already said that both Marvel Studios and FOX can use the characters, so I'm not seeing what the issue/dispute here is.
The problem isn't that FOX is using Quicksilver.
The problem is that they are giving him a pointless action scene role, right after Joss announced he was planning to use Quicksilver as a prominent character in Avengers 2. Also Fox shoehorns in as many X-characters in as they can whether they have personalities or not. This has become a major problem for the films. It feels like FOX is only doing it to spite Disney, giving QS a pointless role and leaving Joss to pick up the pieces.