The 20-year-old was offered a new five-year contract by the Spanish outfit, but Del Nido has revealed that the Spaniard rejected their offer.
"We made him an offer but he did not accept. He wanted out," Del Nido told the club's official website.
"We have reached an agreement with Crystal Palace for about €2m (£1.7m) plus add-ons and a sell on fee. He will travel to England to sign today and have a medical tomorrow."
Palace were victorious in the Championship playoff final last season.