He was signed to a 5-year/$50M deal with $20M guaranteed from the Rams and in his first year with them he had the following numbers:
140 tackles 6 sacks 2 INT's 10 TFL's 10 Pressures
So with those numbers, why are the Rams releasing him and taking a $7.5M dead money cap hit ($5M cap savings)? Early talk is the Rams need cap space and Wagner wants to play for a team that has a Super Bowl chance.
While Dallas could possibly fit that description, there are already reports saying Cincy and Titans are interested and Dallas' worst nightmare: the Eagles may be interested.
If Jerry is serious about winning a Super Bowl he finds a way to sign Wagner as the pairing of Parsons and Wagner would elevate this defense to one scary defense.
That said, keep in mind the Rams played a 3-4 defense with Wagner as the RILB whereas Dallas runs more of a 4-2-5 big nickel defense so would Wagner actually flourish in that style of defense?