col has the correct answer.
Both Barcelona and Madrid were after him, and Barcelona negociated with River Plate while Real Madrid spoke with Milionarios. Legally, the player should represent Barcelona, but this happened during Franco's regime (FC Barcelona's stadium was perhaps the only place where the opposition could speak) and the decision was that the player should play one year in each club, starting with Real Madrid. Of course, after one year, it was easy to keap him in the club.
Next question, then.