I used to think the Rangers should not go over the cap and have it count toward next year. However, I believe I've changed my mind. If the Rangers can get a top defenseman at midseason without giving up too much, I think you have to do it.
If the Rangers can not get a top defenseman at midseason without giving up too much, then I'd consider upgrading at forward. Forsberg, is a HUGE risk / HUGE reward type player. Simply - there is absolutely no other player we can get for zero assets that could have an impact like him. Even if we traded a lot of youth we might not get a player that could have a bigger impact than him. So if we can't get a #1 defenseman for a reasonable price - I think you have to try to get Forsberg if he is willing. I'd take the risk.
To answer the question about salary: I suspect he'll have a very limited number of teams he'll want to sign with. And all will probably be at or close to the cap. So he'll have to accept a contract loaded with bonuses, and it can't go above 7.5% of the cap (3.75 million). I'm unsure if the contract would actually be 3.75 a year, or double that for the full year (but there is only half a year left)
By my calcs - the Rangers are just about at the cap and will pass very little if any to next year. Therefore they are on equal ground with any other team at the cap.