Real Madrid president Florentino Perez claims Inter attempted to sign Luka Modric without paying for the star midfielder.
Modric – who was crowned UEFA Player of the Year for 2017-18 on Thursday – emerged as Inter's primary transfer target following the World Cup in Russia but the Italian side have been unable to lure the 32-year-old midfielder to Milan.
Madrid sent an official complaint to FIFA, alleging Inter contacted the player illegally over a potential move while the Serie A outfit confirmed legal action against La Liga president Javier Tebas, who criticised the club's pursuit of Modric.
Modric also described a report claiming he approached Inter about the prospect of a transfer as "the greatest nonsense in history."
Still unhappy with Inter's pursuit of Modric, Perez was quoted as telling journalists: "I've never seen a club want to buy our 10 without paying.
"It's the first time this has happened to me in my life."
Madrid veteran Modric was honoured during the Champions League draw in Monaco on Thursday.
The Croatia captain, who also claimed the Golden Ball at the World Cup, won the most votes from coaches and journalists as he beat Cristiano Ronaldo and Mohamed Salah to the UEFA gong following his role in Madrid's third successive Champions League success.