This means they will be free to sell Nicola Barella to Chelsea.
The Blues are trying to line-up a replacement for Cesc Fabregas, who has agreed to join Monaco.
However, Chelsea will force Fabregas to wait until the summer if they cannot complete a deal for Barella.
That looks unlikely now after Cagliari's recent success with the Nandez transfer, which was confirmed by Boca president Daniel Angelici.
"We have accepted Cagliari's offer for Nandez because we spoke to the player and he wants to go to Italy," Angelici told Radio La Red.
"What we do not accept is the form of payment and that is what we are working on.
"It will be up to the Italian club to make the effort. At the moment the first instalment of the offer is not enough to pay taxes."