Darren Bent insisted he had to leave Aston Villa after too many "slaps in the face".
Brighton defender Greg Halford has agreed to extend his loan deal from Nottingham Forest until the New Year.
The France Under-20 international has signed a three-year deal at the Stadium of Light and becomes the seventh man to join the club since the end of last season.
He has already joined up with his new team-mates and manager Paolo Di Canio at their training camp in Italy.
A product of Le Havre's youth set-up, Ba broke into the first team set-up at the beginning of 2012 and went on to make 12 league appearances.
He follows Valentin Roberge, Modibo Diakite, Cabral, Jozy Altidore, David Moberg Karlsson and Duncan Watmore into the club, and the list of newcomers could be boosted further within the next 24 hours.