Crawley have rejected £100,000 offer from Notts County for striker Izale McLeod.
Rotherham boss Steve Evans is hoping to add two more signings to his squad before the start of the season.
A statement on the club's official website confirmed that Fernandez made the announcement to his players and the club's directors on Thursday.
Youth team coach Patrick Gabriel will take charge of the team on a caretaker basis.
Nancy prop up Ligue 1 with 11 points, eight adrift of safety, and only one win all season, which came on the opening day against Brest.
Fernandez's last match in charge brought them their first win in any competition since then, a 5-1 success against fifth-tier Dreux in the Coupe de France on Saturday.