The Italian club are continuing to strengthen their squad ahead of the 2012-13 campaign, with the lucrative sale of Edinson Cavani to Paris Saint-Germain having already funded the acquistions of Gonzalo Higuain, Dries Mertens and Raul Albiol.
But if they are to capture the 26-year-old, who scored an impressive 31 goals in 40 appearances last season, the player's representative believes the Stadio San Paolo side will be forced to part with a hefty sum.
Manuel Manzo told Sky Sport Italia: "There's an agreement with the player, but we're missing one with Porto and the club don't want to sell him.
"The only way it could happen in my opinion is that they pay the €40m release clause, otherwise he won't leave.''
Martinez joined Porto from Mexican outfit Jaguares in July 2012 and helped his new club to victory in the Primeira Liga in his debut season.
The Colombian forward has also been capped on 18 occasions by his country and has scored on eight occasions at international level.
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