Sevilla's president has confirmed the move, with Real Madrid having a clause in the Lyon forward's contract affording it the first right of refusal.
Real Madrid have exercised their buy-back clause in the contract of former striker Mariano Diaz, who they sold to Olympique Lyon last summer.
Sevilla president Pepe Castro was quoted by Marca on Tuesday confirming that Los Blancos have decided to bring the Dominican Republic forward back to the Santiago Bernabeu, as they look for competent backup for Frenchman Karim Benzema.
"We are an ambitious club that tries to make ambitious acquisitions, Madrid had a trial option and they told us they were going to exercise it," the president said, adding: "But the player has not told us and we are waiting for him to answer, because he has told us a thousand times that he wanted to play for Sevilla."
Sevilla had already agreed a deal to sign the 25-year-old, although it looks as though Mariano's former club have hijacked the deal with just three days of the transfer window remaining.
Mariano scored 21 goals in Ligue 1 for Lyon last term as he impressed in his first venture away from the Spanish capital, attracting the interest of his former club.
Madrid are particularly keen on attacking reinforcements having lost Cristiano Ronaldo to Juventus this summer, and have only made one signing in that department in the shape of former Fluminese player Vinicius Junior.
Sevilla will now pursue other avenues, with Girona's attacking midfielder Portu at the top of their wishlist. The player has an £18m release clause.
Real Madrid were not expected to bring anymore players in ahead of Friday's deadline, but they have decided to act after hearing the advancements in the deal between Sevilla and Mariano.