Real Madrid signed Racing Santander midfielder Sergio Martinez on Monday for a reported three million euros.
"Real Madrid will pay the full amount set as the footballer's contract release clause," said Racing on social media.
The 19-year-old scored against Villarreal in Racing's LaLiga opener, but is expected to play for Madrid's reserve side at first.
Los Blancos have signed several players this summer, including record arrival Yan Diomande, after failing to win a major trophy for two seasons.
Jose Mourinho's side have started the season in perfect fashion with three wins from three LaLiga matches.