The NFL was ordered to pay back over $100 million to a shared revenue pool with the NFLPA, arbitrator Stephen Burbank ruled last week.
Burbank found that the NFL created its own exemption, pocketing $120 million in ticket revenue over a three-year period, leading to $50 million being detracted from player salaries over this time frame.
The NFLPA filed a grievance in January.
"They created an exemption out of a fiction and they got caught," NFLPA executive director DeMaurice Smith said, according to Matthew Futterman of The Wall Street Journal.
By the NFLPA's calculations, the salary cap ought to increase by $1.5 million per team for the 2016 season due to the NFL's payment.
The NFL refused to disclose the figure they paid back to the shared revenue pool, but league spokesman Brian McCarthy said their erroneous accounting was due to "funding of stadium and construction projects."
"People have become accustomed to how we protect our rights when it comes to player discipline," Smith said. "We are equally diligent when it comes to getting our share of revenues."