However, the agency insists he will continue to work for Fallon and will retain his job title. There is no time limit on the duration of his trip.
Cabral, who joined the agency from Mother in 2004, has been responsible for some of Fallon's most awarded work, including the Sony Bravia "balls" and Cadbury "gorilla" spots.
His decision was announced in an all-staff meeting on 11 September.
"We're thrilled for Juan and looking forward to the next chapter in our working relationship," Laurence Green, the Fallon chairman, said. "Fallon London's briefs are increasingly international and Juan remains knee-deep in a good number of live projects."
"We are always open to new ways of working," Karina Wilsher, the Fallon managing director, said. "The culture is centred on talent, wherever they are."