Argentine goalkeeper Caballero has been strongly linked with a winter move to Manchester City to be reunited with former Malaga coach Manuel Pellegrini.
England number one Joe Hart's recent axing from the City side following a patchy run of form has also come amidst a backdrop of rumours that Pellegrini is keen to bring in Caballero.
Teenage midfielder Darder has also been linked with a move to the Premier League, with reports claiming Tottenham Hotspur are following his progress.
However, Casado does not want to lose either player unless the release clauses are triggered, with Caballero's standing at €20million (£16.9m) and Darder's at €40million (£33.8m).
"We will not sell Willy Caballero or Sergi Darder in the January market," Casado told radio station Cadena Ser.
"He (Caballero) will not leave unless someone pays his release clause and he is in agreement. At the moment we are focused on building the squad."
Darder only signed a new contract at Malaga last week after being promoted to the first-team squad this season and he brushed off rumours about his future.
"I am only focused on my work in Malaga," he said. "The future? I am calm and my desire is to progress in world football.
"When I first started pre-season training, I never knew I'd have such an amazing opportunity.
"This is what I've been working for all this time. Malaga really has given me everything.
"I'm so thankful to the club for where I am today, and I'd like to remain here as long as I can. I'll always be grateful to Malaga."