The contract is reportedly worth some €2.5 million per season, a significant raise over what Koulibaly made in his original Napoli contract signed a little over two years ago. During those two years, though, the 25 year old defender went from being an extremely raw prospect to being one of the best defenders in Serie A and in the upper tiers of defenders across Europe, and he's definitely earned a major raise. Reports vary as to the existence of a release clause, but if there is one in the contract, it's generally agreed to be worth around €70 million and can only be activated by non-Italian clubs -- a lesson Napoli learned thanks to Gonzalo Higuain's defection.
Of course, the way Koulibaly and Napoli got to this contract was a contentious one, with Koulibaly rejecting multiple contract offers in the summer and airing largely contrived grievances in the media in ways that absolutely infuriated both Aurelio De Laurentiis and the Napoli fanbase. That sparked a long transfer saga that saw Koulibaly linked mostly to Chelsea and Everton in the Premier League, but Napoli rejected every offer that came their way, knowing they had to keep their best defender in order to meet their goals for this season.
Once the transfer window closed and Koulibaly was still with Napoli, attention turned from a transfer to a new contract, and less and a month later here we are with Koulibaly putting pen to paper. It's a good end to this story, and one that should raise the spirits of the Napoli fanbase, knowing that the best defender the club has had in some time is tied to the team for what should be the best years of his career.