Interest is high in the Napoli defender and he has been heavily linked with a move to a top European club in recent months.
Napoli president Aurelio de Laurentiis is not prepared to let him leave on the cheap, however, and is demanding a world-record fee for a defender, claim Gazzetta dello Sport.
Koulibaly has made a name for himself since moving to the south of Italy from Belgian side Genk in 2014.
Because of this, Napoli would be far less likely to part with Koulibaly, even if clubs are ready to break the bank for the defender.
According to Il Mattino newspaper Bayern Munich are offering €110m, while reports out of England suggest it will take a blockbuster offer of over €150m for de Laurentiis to sell Koulibaly. And that price might be too rich even for a club as big as Manchester United.