In a significant coup for the Broncos and John Elway, Peyton Manning is reportedly set to accept a $4M pay cut, down to a $15M salary for 2015. Manning's original deal also calls for him to make $19M in 2016; according to Mike Klis, that portion of his deal will remain unchanged.
Of course, this flies in the face of every claim that Manning is a selfish, greedy bastard, and less of a team player than Tom Brady, who has never in his career accepted a pay cut. Per Klis, the Broncos had sought an even larger reduction.
If it's a simple pay cut, then Denver's available cap space would increase to a shade over $19.5M. However, a more nuanced restructure could easily free up more space for 2015 by shifting some of the caponomic burden to future (possibly voidable) seasons.
According to Mike Florio, Peyton can earn back the $4M in lost pay via incentives. It's unknown whether they're of the likely or not likely to be earned variety, but given the whole point of this exercise, we'd assume they're the latter. Klis reports the incentives are team-based, which we can only take to mean #winning.
As noted by Florio, it's a safe assumption that Tom Condon was unable to find another team (ie Houston) willing to pay Manning $19M or so to play in 2015.