Sabaly, 23, had earlier extended his contract with the French champions until 2020 before agreeing the switch to the south-west on a one-year loan.
Sabaly has spent the last three campaigns out on loan, with two years at Evian being followed by a season at Nantes.
Bordeaux have also extended the contract of France under-21 international forward Enzo Crivelli until 2019, although he will now move to Bastia on loan for the season.
It has been a summer of rebuilding at Bordeaux, who have appointed a new coach in Jocelyn Gourvennec, previously with Guingamp, and have already signed midfielder Jeremy Toulalan from Monaco, Jeremy Menez from AC Milan and Guinea forward Francois Kamano from Bastia.