The Bills converted $11.7MM of Stefon Diggs‘ base salary into a signing bonus (Twitter link via ESPN.com’s Field Yates). In turn, the Bills now have $7.8MM in extra cap space for 2021, though that amount will be pushed onto the 2022 books. 
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The Vikings signed Diggs to a five-year, $72MM extension in July 2018. However, Diggs’ $14.4MM-per-year salary was too much for Minnesota given their cap crunch. The Bills traded for him last year, surrendering a first-round pick in the swap. The Bills’ thrilling run to the AFC Championship Game probably wouldn’t have happened without Diggs, one of the league’s most exciting wide receivers.
The Bills gave Diggs a small pay bump/advance on his money upon acquiring him, so he was happy to reciprocate this week. Despite all the changes, he remains locked up through 2023 on a manageable contract. Diggs, who won’t turn 28 until November, registered 127 catches and 1,535 receiving yards last year to lead the NFL.
Possibly dumb question, but what’s the limit on how much of someone’s salary you can convert to a signing bonus to create cap space? And is there a limit to the number of players you can do this for? Seems like a team could just convert their whole roster’s salaries to signing bonuses and create a ton of space.
The amount is the veterans minimum salary. So they can convert however much they want up until the min contract
The reason teams don’t do this is because it’s kicking the can down the road. It will create future cap issues because signing bonuses are spread over the entire length of the contract where base salary is only for single season. So if you take that 12 million he was due this season and spread it over the next 3, his cap number is now 4 million more than it was but the actual cost remains the same. So he may only make 10 million next year but will count 14. Do that too many times and you have too much dead money on the cap
The bears are a good example of a team who kept doing this and now as a middling at best unit who cut a pro bowl corner have 200k in cap space
Extensions with fake years is where you really circumvent the cap
I dont think there is technically a limit as you can also extend the contract (ala brees) or pay more now and make the cap less in future years (ala Kaepernick back in the say).
They could also lower the base salary and make it guaranteed too. IE John doe has a 5 mil base salary/cap hit so the team wants to cut him unless he takes less money but it is guaranteed. So John salary changes from 5 mil non guaranteed to 1 mil guaranteed.
Usually though most teams will convert this year entire base salaries to signing bonus and kick it down the road some.
this amount is damn close to ertz cap hit