Various English publications have reported that the Magpies are looking for a fee close to £35million for the midfielder who helped his country reach the final of the recently concluded Euro 2016 tournament.
According to Mondial Sport player agency, Sissoko, who has attracted the interest of Arsenal and Tottenham, was promised he would be able to leave the club for a much lower fee following the Magpies relegation from the English Premier League last season.
"We do not understand. It was agreed that Moussa stayed one more season at Newcastle to help the team," the player's representative told French publication Foot Mercato.
"He did the best he could. The club has always said Moussa could leave for between £15.2million and £21.1million maximum.
"We hope that Rafael Benitez and the president will be more reasonable about Moussa's future.
"He has always been loyal, he has always respected the club, the fans. There are clubs who came to us and the club. We are fully aware of their approaches.
"We want everything to go well."
Since joining Newcastle from Toulouse in January 2013 for a fee of £1.5million, the 26-year-old has scored 12 goals and registered 19 assists in 133 appearances.