Leno isn't good but releasing him didn't improve your roster with whatever money you saved. What is
@Pegger saying here, its actually 6M?
I'd recommend going to
www.overthecap.com. It's a great resource not only for team/player specific contracts, but they also produce a lot of articles that are really well informed that follow the same logic that GMs have to do. Another good resource is
www.spotrac.com. If I'm being picky it has better 'data', but isn't as user friendly in terms of learning how the cap works and getting articles that apply cap concepts to day to day things.
I'll try to be brief. Leno's cap number was $11MM if he was still on the roster. Cutting players can create something called 'dead cap space'. This is when certain payments like signing bonuses and restructurings happen where the player gets all the money up front, but the team can spread their cap hit of those payments over following years. If the player is cut/traded before those payments are accounted for it results in the team having to account for them typically in the same year the player is no longer on the team.
For Leno the Bears cut him with a post June 1st designation. This means they can take what the full dead cap space hit for 2021 be and spread it over a 2 year period. Due to signing bonus/restructuring that happened within his contract the 'dead cap space' will be $2,294,000 this year and $2,788,000 in 2022. So adding those two together it's ~$5MM, so the total 'savings' of cutting Leno is his 2021 full cap number minus the dead cap space, so 11 - 5 = 6.
If we want to get real technical cutting him 'saved' ~$8.7MM in 2021 ($11M - $2.294,000), but will come at a future cost of $2,788,000 to the 2022 salary cap.
Some new to the salary cap might ask, why would a team structure things in that manner? The answer is that it's the norm in the league. Let's look at Dak Prescott. His signing bonus was $66MM with a base salary of $9MM for 2021. There's no way the Cowboys could have one player account for $75MM (66+7=75) in 2021, so based on the rules they could equally spread the $66MM signing bonus over 5 years, being $13.2MM each year, meaning when you add his salary into the mix his personal cap number shouldn't be above ~$47MM for any specific season.
For Dak it makes sense because he gets $75MM in 2021. For the team it makes sense because they can spread the largest part of his contract, being the signing bonus, over 5 years.