Let's start at the beginning. DeSean originally signed a 5-year, $47 million contract during the 2012 offseason. This is how that contract shaped up:
|Year||Cap Value||Base Salary||Proration||Other Bonus||Dead $ If Cut|
2014 is bolded because that was obviously the current year of the deal when Jackson was released. Prior to Jackson's release, the Eagles had somewhere around $16 or $17 million in cap space, depending on the source. OverTheCap had the Eagles at $16,253,669, which was the 8th highest amount in the NFL at the time.
By releasing Jackson without a post-June 1 designation, the Eagles created $6 million in dead money for this year. As PFT explains:
[The $6 million is the] remainder of the $10 million signing bonus he received in 2012. He keeps that money free and clear, part of the $18 million he made in two years under a five-year deal.
If the Eagles had used the post-June 1 designation, Jackson would have counted $2 million against the cap in 2014 and $4 million in 2015. However, his full cap number of $12.75 million would have remained on the books until June.
So, in simple terms, the Eagles generated $6.75 million in cap space and saved $10.75 million with Jackson's release. That means they have somewhere around $22 to $24 million now, according to OverTheCap and EaglesCap. Based on either number, the Eagles now have the fifth most salary cap room in the NFL. But what can they do with all of that money? I explained this recently:
It would make sense for the Eagles to [roll over their cap space from 2014] considering they have a number of young players who could be up for contract extensions such as Nick Foles, Brandon Boykin, Mychal Kendricks, and Fletcher Cox. Foles is especially relevant here because of his position. If he continues to play well, he will be in for a big pay day. Quarterbacks make crazy good money.
Once again, the Eagles didn't really have to release Jackson because for money reasons. This is just a relevant side effect of the Eagles' major decision.