Rangers boss Stuart McCall says he feels sorry for Gael Bigirimana, but admits he should never have been signed.
Chris Gunter will stay at Reading, after the club took up the option to extend his deal by another year.
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 U-19 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."