Napoli president Aurelio De Laurentiis has placed a €100 million (£88 million) price tag on Marek Hamsik.
Hamsik, 23, recently admitted he would welcome a move to AC Milan, one of the clubs reportedly keen to sign him this summer.
"The indecent proposal I referred to (earlier this month) would be of €100 million," De Laurentiis said. "Otherwise, Hamsik will remain here."
Hamsik, who joined Napoli from Brescia in 2007, is under contract with the Serie A club for another four years.
He scored 11 goals in 37 league appearances to help Napoli finish third last season and qualify for the Champions League.