Real sold Mariano to Lyon in 2017 and are on the verge of bringing him back to help fill the gap left by Cristiano Ronaldo's departure to Juventus.
Sevilla had agreed a deal with Mariano but Madrid activated their first-option clause included in the transfer that took the 25-year-old to France.
"We are an ambitious club and we try to make ambitious signings. Madrid had a first option and they have told us they will execute it," Castro told reporters Tuesday.
"But the player has not told us anything and we are waiting for him to respond because he told us a thousand and one times that he wanted to play for Sevilla."
Mariano joined Lyon for 8 million euros ($9.2 million) and Madrid are expecting to re-buy him for around 25 million euros, according to reports in Spain which also say Real still own 35 percent of the player's sporting rights.
"Without doubt another forward will arrive," added Castro, who said Madrid striker Borja Mayoral is one of a number of players the Andalusians have looked at signing.
Mariano, who played one friendly for the Dominican Republic before retiring in the hope of a Spain call-up, scored once in eight league appearances for Real Madrid but hit 18 in 31 for Lyon in Ligue 1.
(Reporting by Rik Sharma, Editing by Ed Osmond)
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