Otamendi, who played for Diego Maradona's Argentina at the World Cup in South Africa, will move now for 4.0 million euros, they said on their official website describing the deal as "a novel operation."
The deal allows Velez to decide in a year's time whether to sell the other 50 percent of the central defender's transfer rights to Porto for another four million euros or keep it.
The terms are that Velez will have the right to make such a choice with a view to possibly making a bigger profit on their 50 percent if one day the player moves on from Porto for a higher transfer fee.
"The decision depends entirely on our club," Velez said, adding if they opt to sell off the other half they will still get 10 percent of any future increased value on the player and five percent for developing him.
The 22-year-old Otamendi, who will have a medical at Porto before signing for the club, spent 15 years at Velez.
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