The problem isn’t that he’s sacrificing the money, because he probably isn’t. It’s now a possible loophole:
1. Player retires, “sacrificing” their pay to free up the cap.
2. Player gets a job with the club, paying a large amount over several years.
The club can offer a high paying job to entice the player to retire and sacrifice his pay from the cap, but the player is still getting paid by the club.
It’s like us telling Foran if he retires and gives up his cap pay, we’ll give him a $500k a year job to make up for the money that would have been in the cap.