CSKA Sofia are suing the Premier League champions for money they claim is owed for the development of Dimitar Berbatov.
The Bulgarian club have filed a lawsuit with Fifa and hired the international sports lawyer Professor Ian Blackstock to fight for what is known as a "solidarity" payment out of Berbatov's £30.75m transfer from Tottenham Hotspur on 1 September last year.
It was in 2001 that FIFA introduced this payment so that clubs who had developed players from a young age were compensated when the player had big-money moves later and CSKA claim United owe them around £475,000. CSKA and Manchester United have been unable to settle the dispute and CSKA have now asked for Fifa to intervene, with a hearing likely to take place in Zurich towards the end of June.
Berbatov, capped 68 times by Bulgaria, left CSKA for Bayer Leverkusen in 2001 and subsequently moved to Tottenham for £10.75m in the summer of 2006.
Meanwhile, Cristiano Ronaldo could miss the start of pre-season training at Manchester United next month after withdrawing from Portugal's squad for Wednesday's friendly against Estonia with a suspected hernia injury.