Jackson Martinez's agent has warned Napoli that they must pay his client's €40 million [£34.8m] release clause to secure the Porto striker's services.
Partonopei boss Rafa Benitez has confirmed that his side are interested in bringing another striker to the club this summer, with the 26-year-old widely linked with a move to the San Paolo.
And his representative, Manuel Monzo, has confirmed that his client has agreed personal terms with the club, but stressed that they are yet to strike a deal with the Serie A club.
"There's an agreement with the player, but we're missing one with Porto and the club don't want to sell him," he told Sky Sport Italia.
"The only way it could happen in my opinion is that they pay the €40m release clause, otherwise he won't leave."
Porto have already lost Joao Moutinho and James Rodriguez this summer.