CA Union coach Leonardo Madelon told a press conference the club has already secured 27-year-old Cabrera's services, according to Argentinian media outlets El Litoral, Uno Santa Fe, TNT Sports and Aire Digital.
"We already have everything arranged with Javier Cabrera as the first reinforcement," Madelon said.
"He played as an 'extremo' for Argentinos Juniors, he has fast characteristics, is light and plays wide.
"He is currently in Australia and will have to adapt to the Union system."
Cabrera only joined City before the start of the current A-League season, signing a two-year deal, but the Uruguayan is reportedly keen on a return to South America.
Union play in Argentina's top league, the Primera Division, where Cabrera spent two seasons with Argentinos Juniors.
The club will also play in the Copa Sudamericana after finishing eighth last season.
Free agent and former Johnny Warren medalist Marco Rojas is being touted by The Sydney Morning Herald as a potential replacement for Cabrera at second-placed City.