The 18-year-old defender has spent the last two seasons in Ligue 2 and represented his country at U16 to U18 level.
Club president Jean-Pierre Louvel told Radio Monte Carlo: "We have reached agreement with Sunderland for the transfer of Mendy.
"It's now up to him to reach an agreement with them. We are okay."
The left-back has also been linked with Arsenal and Lyon and Louvel did not rule out Remi Garde's men sabotaging the deal.
"We were close to an agreement with Lyon, it has not happened but you never know," added Louvel.
"Right now, the only club we are in agreement with is Sunderland and it is now up to Benjamin Mendy to make his choice."
Wearside have played down reports that a deal is at an advanced stage for Mendy but confirmed he is a target, with the club having been without a specialist left-back for some time.