Dallas Stars

Why does Detroit do this? They get an upgrade for their 4th line centre position. Faksa is good penalty killer and good at faceoffs. Faksa is overpaid right now but with the little bit of retention, value for cap hit is a lot more reasonable.

This deal is made so Dallas can save cap space to add a player like Tanev without having to send out Hakanpaa.
Rasmussen can replace what Faksa brings but for cheaper. He hasn't played that well in Lalonde's system but could excel in Dallas. Dallas also gets the younger player who is still developing. I believe they are a well rounded enough team that losing Faksa for Ras won't be noticed that much compared to the upgrade on defense Tanev could bring.
In order for it to be worth while for Detroit, there has to be a bit of cap savings on Faksa. Stranges and a 5th next year for a bit of added value to Detroit.
Dallas saves 1.14M in cap space this season. Rasmussen could likely be re-signed for a similar cap hit.

Dallas adds to their defense by acquiring Tanev. Not only will he make their defense better but he could be a good mentor and partner for Harley and if he re-signs in Dallas then Bischel in the future.
I would expect his extension, if he chooses to sign one, would come in at about 3.25 x 2 years.

A 1st and prospect might be a bit rich but I expect there to be quite a few teams interested in his services.
By moving out Faksa, they can keep Hakanpaa and Lundkvist on the team.