The 27-year-old joined the Black Cats in 2016, when they were in the Premier League, but spent last season on loan at Roazhon Park, making 24 appearances in Ligue 1.
Khazri seems certain to leave Wearside after two-and-a-half seasons, and Letang has urged the Tunisia international to make a quick decision ahead of the World Cup.
"The ball is in the hands of Wahbi," L'Equipe quotes the Rennes president as saying. "He has a proposal from the club which, for Rennes, is a very good offer.
"We compete with clubs that have a larger financial footprint. But we can not play with the institution and we need to know quickly if the player wants to be part of this project."
Khazri scored 11 goals on loan at Rennes en route to a fifth-place finish in the table but suffered a thigh injury in April and missed the end of the season.