Le Havre announced on Thursday that they had accepted an offer from the Black Cats for their promising 18-year-old left-back.
They expected a deal to be done with Paolo Di Canio's side, with the matter having been left in the hands of Mendy and his representative.
No transfer has been put in place, though, as a number of other clubs remain in discussions.
Cardiff City are also keen on taking Mendy to the Premier League, while Lyon are looking to keep the France U19 international in his homeland.
"Nothing has been sorted with any club," Mendy's agent, Sylvain Chavanon, told L'Equipe.
"Sunderland are one of the teams that we are in talks with.
"There is also Lyon. Cardiff have spoken with us, but we don't have any news at the moment."