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The NFL players union said they would fight the penalty clause that could force Ricky Williams to pay back over $8 million to the Dolphins, the Miami Herald reported in Thursday's editions.

''It's our position that these penalty clauses that teams have put into contracts are unenforceable, depending on state law," NFLPA attorney Richard Berthelsen told the newspaper. While not familiar with the Williams contract, Berthelsen said the players association would likely file a grievance if the team pursued action against the retired running back.

Williams stunned the Dolphins when he informed them last Friday of his decision to retire. The former Texas star, who has been traveling for the past week, was in Boston on Tuesday and may return to South Florida by the end of the week, said his agent, Leigh Steinberg.

Because of penalty clauses included when Williams' contract was reworked two years ago, the team could try to recoup $5.3 million in incentive money, said sources familiar with the terms who requested anonymity. The Dolphins also could seek $3.3 million of the $8.8 million signing bonus Williams received when he joined the Saints in 1999, the sources said.

In the union's view, Berthelsen told the Herald, a team could go after cash paid in a signing bonus. But in Williams' case, the Saints -- not the Dolphins -- paid the signing bonus, in 1999. Williams' deal still has approximately $2.5 million of a pro-rated signing bonus left.

The Dolphins said they've made no decision about whether to pursue the matter. Williams' contract expires in 2006.

For his part, Williams decided to retire aware of potential consequences, financial and otherwise, Steinberg said.

"He's very much at peace and excited about embarking on the next phase of his life," Steinberg said. "I don't anticipate him changing his mind, but history is replete with examples of athletes who had second thoughts at later dates."

Williams was to earn at least $3.6 million this year, with incentives possibly pushing that total as high as $6 million.

The Dolphins acquired Williams before the 2002 season. He led the NFL that year with 1,853 yards rushing and broke nine team records. Last season, he ran for 1,372 yards.

Information from The Associated Press was used in this report.