I maybe wrong cause I'm no salary cap guru but I don't think Decastro new deal is going too bring any salary cap room anyway. He isn't going to take less then what he is guaranteed this year correct??
Because the deal will be 5 years and whatever signing bonus they give him can be amortized over those 5 seasons it will almost definitely be less than his current $8.07 million number.
For instance even if they give him $18 million THIS YEAR and make that into a $3 million salary and $15 million signing bonus, that would be only a $6 million salary cap hit for 2016. Obviously, they don't HAVE to use a signing bonus or amortization, but the Steelers seem to have no qualms pushing any/all money into future seasons and playing the credit card game. In some ways that makes sense.... a $1 on your salary cap books now is actually worth more (by percentage) than a $1 in 2017 and so forth. The only thing amortization does is limit the Steelers ability to part ways during the contract (and thus give the player some negotiating power to stay on the roster), but the Steelers rarely do that.
Although it has happened - Mahan, Woodley, Starks, C. Allen all being examples of players only making it 2 years into a 4 or 5 year long term deal (and thus having the Steelers eat some fairly large "dead money" accounting corrections).