Casado can leave the team either on loan or in a permanent transfer. If he's sold permanently, Barcelona will insist on having a buy-back option. This would allow the club to re-sign him in the future if needed.
There are rumors that Casado might join a team in England. Tottenham is reportedly interested in signing him.
Barcelona wants to sell Casado to balance their budget and make room for new players, including Marcus Rashford and Joan Garcia.