The National Football League has been ordered to return what its union calculates is more than $100 million to the pool of revenue that it shares with its players.

The ruling, handed down last week by arbitrator Stephen Burbank, found that the NFL owners had mischaracterized what Players Association officials say is roughly $120 million of ticket revenue during the past three years by creating an exemption that had the effect of keeping about $50 million in salary out of players’ pockets. The NFL Players Association, which...