Real will exercise their right to first refusal on the player, despite the Andalusian club agreeing a deal with Lyon.
"We are an ambitious club that tries to make ambitious deals," Castro said.
"Madrid had a buy-back option and they have told us that they are going to exercise it.
"But the player hasn't told us anything yet and we are waiting for him to respond because he's said a thousand times that he wants to play for Sevilla," he explained.
The Sevilla president went on to speak more about the fast-moving situation.
"Sevilla have acted how we always act. We reached agreement with the French club," he said.
"The player told us that he only wanted to play for Sevilla. The reality is football is football. They have a clause and they've triggered it.
"We don't have anything to ask Madrid because that's Lyon's obligation. No problem, we continue. I'm not annoyed with anything, this is football. I'm not worried."