I'm thinking 7 mil saving for us (plus the draft pick money) would end up luxury wise be a savings of over 10 million, which we could use to sign Posey. The question isn't whether we have the ability to sign Posey, it's how much of a luxury tax hit we're willing to take to get it done. This trade would lesson the hit.
Saving luxury tax money has no effect on our ability to re-sign Posey, beyond the question of how much of a hit Wyc is willing to take on the luxury tax. Even if we were able to dump Scalabrine's salary, we'd still be well over the salary cap and thus only able to offer Posey the mid-level exception. Fortunately, just about every other contending team is in the same position, and since Posey is well-known to be an excellent role player on a contending team, he's unlikely to sign with a team that's under the cap, so he's unlikely to get more than the mid-level exception. Which means the Celts will probably be able to get him back.
As for Scal, the best use for him is probably to wait a year and then move his expiring contract in a deal for something useful. We could take back a player making a similar amount as Perkins, so that might net us a decent role player from a team looking to cut salary. Or if we package him and Ray Allen we'd be able to take back over $27 m. in salaries, which is enough to acquire any player in the league (whether we'd have enough sweetener to make it worth some team's while is another question). Or we could package him with Perkins and a future pick or three, allowing us to take back over $9.5 million, which might enable us to grab a young star who's on the verge of free agency and isn't going to re-sign with his current team (no, probably not LeBron). Keep in mind that next year KG's salary drops by nearly $8 million, so they'll have some relief from the luxury tax if they want to add some salary. So next off-season is the one where there might be a chance for Danny to make a splash on the trade market, but there's not a lot he can do this season.