The Blades claim Tevez was not eligible to play at the end of the 2006/07 season, when he played a key role in West Ham's top-flight survival at their expense.
Past and present players have held discussions with Farnell, of the Manchester-based firm IPS Law.
It is understood they could claim for lost earnings running into millions of pounds.
The latest development comes in a troubled week for West Ham.
New boss Gianfranco Zola saw his team, who are sixth in the Premier League, knocked out of the Carling Cup by Championship side Watford.
The tribunal ruling came as the club is still searching for a new shirt sponsor following the collapse of travel firm XL Holidays.
Striker Carlton Cole was arrested by police in central London at 0425 BST on Tuesday on suspicion of drink-driving.
And forward Dean Ashton could be out for a month as he requires ankle surgery.
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