When the National Football League adopted a liberal free agency rule in the early 1990s, allowing players to more easily jump to new teams for lucrative deals, no superstar was expected to earn more than future Hall of Famer Reggie White, an intimidating defensive lineman whose contract with the Philadelphia Eagles was expiring. “Everybody said that free agency in football is not going to work because everybody will go to New York or Los Angeles, and nobody will go to Green Bay,” said Kenneth L. Shropshire , director of the  Wharton Sports Business Initiative  and author of a forthcoming book on negotiating and sports.…