I think financially receiving nothing for a player that has a current value of £20m is a big loss to a club that clearly has had financial issues. I appreciate the counterargument that if we stay up it's worth more than that amount, but we're unlikely to stay up with or without N'diaye.
Look at Sancho, I guess even though City are loaded there must be frustration that they received nothing for a player that went on to sell for £80m (Pogba the same for Man Utd - albeit they were the team that ended up paying that big fee to get him back

)
The best outcome for me is that he signs a 2-year deal giving him a huge pay raise with a buy-out clause for another team to take him for £20-25m. He might only play until January for us on this new deal but it would help him establish himself as a regular premier league player.