Real Madrid are regarded as favourites to sign the young midfielder, whose clause is set at €45m.
Brito told ESPN Argentina: "If the player wants to leave, he goes, he executes the clause and there's not much to do.
"The clause, within the employment contract, gives the player the right to leave for free elsewhere. The club can't handle it: if the player decides to leave, executes the clause, there's not much they can do.
"People often talk about why they didn't raise his clause much more. It's not that it didn't occur to me. Representatives ask that it not be exorbitant; it's designed so that players can leave at a value that's profitable for the club."
Brito added, "Mastantuono is one of the most important arrivals in the country in the last 20 or 30 years. We must respect the contract we have, defend the club's interests down to the last cent, and try to retain the player as long as possible."