Lyon fans on r/soccer seem pretty confident that Michel Aulas wouldn't consider selling at any price. Their 'big' sale for this year is Fekir - they have Champions League football and healthy finances, so they don't need to sell anyone else this summer.
And, even in the unlikely event that they did, N'Dombele would be probably the last player they'd sell. Apparently he's destined to be the best box-to-box midfielder in world football very soon - as good as Pogba. And, to be fair, the Youtube compilations are very impressive - he has a fantastic passing range, and seems about as good at dribbling, defending and physicality as Dembele is.
However, that's all I've seen of him, so I can't say if he's actually that good. But, if this accurate, we're either talking unprecedented money for a 21 year-old from our perspective (75m+ or thereabouts, maybe), or there's some snag in his contractual situation (he's actually on loan at Lyon from Amiens, although the buy-out clause has apparently already been activated).