Last season, Antoine Griezmann had the highest salary under Messi. If on a new contract Messi is willing to be paid a salary reduced by 50%, then Antoine Griezmann becomes the player with the most expensive salary at Barcelona. To comply with La Liga's financial fair play rules, Barcelona should earn new revenue or reduce its expenses. Since Barcelona has chosen to extend Messi's contract, one of their best options is to sell Griezmann or loan him out to reduce their wage cap.
Griezmann is not very successful in Barcelona. He did not perform as well as Barcelona had hoped. In this situation, Barcelona is not interested in keeping Griezmann in the team by paying so much salary. Aguero could be included in the squad as a replacement for Griezmann.