The 24-year-old Frenchman, who spent the second half of last season on loan to Spanish club Real Betis, has attracted interest from Betis, Celta Vigo and Elche in recent months.
But despite agreeing a fee - understood to be in the region of £2million - with the Black Cats some time ago, Betis were unable to table personal terms which were acceptable to N'Diaye.
However, it is understood a further round of talks is due to get under way with no other suitor having pushed home their interest.
Sunderland are keen to offload the player, signed in January 2013 by then manager Martin O'Neill, as they re-shape their squad after last season's remarkable fight for Barclays Premier League survival.
They have already signed seven new players and have high hopes of adding to that list as sporting director Lee Congerton and head coach Gus Poyet focus on their remaining targets with the closure of the summer transfer window looming.
However, the likes of N'Diaye, fellow midfielder Cabral and central defender Modibo Diakite, none of whom have been given squad numbers for the new season, are expected to leave.
The rebuilding plan is being complicated somewhat by the Black Cats' pursuit of top target Fabio Borini, who remains frustratingly out of their grasp.
The Borini camp has indicated the Italian wants to fight for his place at Liverpool this season, having enjoyed a successful campaign at the Stadium of Light last time around, and the collapse of Brendan Rodgers' move for QPR frontman Loic Remy has weakened Sunderland's armoury.
Poyet has been bullish about getting his man in recent days, but the point where the club will have to admit defeat and move on to other options should no deal be struck is fast approaching.
Source : PA
Source: PA