Diario de Noticias says Real filed the case back in May but Benfica are yet to be called to answer.
Benfica sold Garay to Zenit in 2014 for €6 million, three years after signing him from Real Madrid. Madrid received half of that sum (€3 million) thanks to a sell-on clause included in the original deal but believe that the final transfer fee fell significantly below Garay's true market value. Bayern Munich were said to have tabled a bid of up to €15 million to rival Zenit's.
Garay played in Russia for two years before joining Valencia this summer.
Real Madrid stated in their case that Garay's true value was between €10 and €12 million, double the sum he was sold for. The defender spent two years in Russia before moving back to Spain this summer in a deal said to be worth around €20 million.