Mari has been on loan at Arsenal since January on a paid loan deal, which Arsenal now have to make permanent.
The deal should earn Flamengo around €16m for Mari, who they signed from Manchester City for €1.3m last year.
Speaking to the Brazilian press on Monday, Braz confirmed what has been reported in recent days.
“Mari was sold, there is no question about that. Sorted,” said Braz. “At the right time, the numbers will be on Flamengo’s balance sheet.”
Mari has managed only two Premier League appearances for the Gunners so far plus one FA Cup run-out.
In Arsenal’s latest match against Mari’s former club Man City, the 26-year-old Spaniard sustained an ankle injury.
Mari is not expected back until November, but will be a part of Mikel Arteta’s squad next season.