The court identified potential illegal activities with criminal implications, including a provision stating that a commission of four million euros per year must be guaranteed to Kosmos.
A document dated Thursday from a court in Majadahonda outside Madrid said "the declaration of Mr Gerard Pique as an investigated party is agreed on March 14".
Rubiales, who faces possible jail time for his forced kiss on star forward Jenni Hermoso in 2023, has always defended the legality of the deal to take the Super Cup to Saudi Arabia.
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Pique has also insisted that everything was "legal", saying he was "proud" of the deal.
(Except for the headline, this story has not been edited by staff and is published from a syndicated feed.)