The Ligue 1 side will purchase all of Cubas’ sporting rights after the sides agreed terms today.
Lawyers of both clubs signed the paperwork to finalise the transfer, which is now missing only an announcement.
Boca Juniors, the club which developed 24-year-old Cubas, will receive a €140,000 solidarity payment.
After playing for Argentina at under-20 level, Cubas chose to represent Paraguay, the birthplace of his father.
He won his first and, to date, only cap for Paraguay against Saudi Arabia last November.
The defensive midfielder already has some European experience from a brief loan spell with Italy’s Pescara in 2017.