That will mean a significant profit for Porto on a player they signed a year ago from Sao Paulo in his homeland.
Zidane made a surprise return for his second stint as Madrid manager on Monday, 10 months after stepping down from the role after guiding the Spanish club to a record third successive Champions League title.
Real Madrid have completed a deal to bring Porto defender Éder Militão to the club this summer.
Militao has netted three goals for Porto this season, including one in the Champions League group-stage win over Schalke.
Jussie Smollett Enters Not Guilty Plea
Smollett has been suspended from the show and was written out of the final two episodes of this season. A grand jury indicted Smollett on 16 felony counts of disorderly conduct for filing a false report.
Real Madrid entertain struggling Celta Vigo on Saturday on Zidane's return to the Bernabeu and he said he was "emotional to be able to return to the Santiago Bernabeu Stadium".
"This is the right moment for all involved - for me, for the squad, for the club".
The defeats ended their interest in the Champions League and Copa del Rey - and left them 12 points behind leaders Barcelona in La Liga.
"I have never put Real Madrid to one side, not even for a moment", said Zidane.
In La Liga, Real Madrid are now in third place, 12 points behind the leaders Barcelona.
Real Madrid president Florentino Perez has been keen on the Brazilian superstar for many years and even held talks with the Paris Saint-Germain owner Tamim bin Hamad Al-Thani last year about a transfer.