The Blades have submitted a claim for £30m in loss of revenue, and players directly involved in demotion to the Championship hope to follow suit with a separate action.
United's players would have been entitled to a substantial bonus for keeping the club in the Premier League in the 2006-07 season and, in line with many contracts, some suffered pay cuts of between 30% and 50% when they were relegated.
The affected players, through the Manchester-based sports law specialists IPS Law, are seeking to recoup those losses from West Ham.
Paul Fletcher, of IPS Law, said: "All clubs, players and agents are governed by the rules of the Football Association and, in the event of any dispute between these parties, the appropriate forum is for the aggrieved party to bring a claim under rule K of the FA rules. This leads to arbitration proceedings and would be heard in front of a panel of one or three experienced arbitrators."