The Scottish Premier League club are battling against the ruling from football's world governing body FIFA.
Casalinuovo moved to Tannadice in 2009, scoring seven goals in 25 appearances, before going out on loan to Raith Rovers and then joining Hamilton Academical in 2010.
United chairman Stephen Thompson said: "It's been all the way through FIFA, we've now appealed to the Court of Arbitration for Sport.
"You're talking about a couple of hundred thousand pounds. That's hard, it's a lot of money.
"It's in the past and I'm not going to blame anybody for anything, because I don't really know who was to blame at the time."
Casalinuovo scored eight goals for United before joining Hamilton for the 2010/11 season.