Tottenham had struck an agreement with Everton on Friday night for Ndiaye, 26, in a deal that would have seen Richarlison, 29, move the other way to Merseyside as part of a separate transfer.
But that arrangement collapsed when the Brazil forward could not agree personal terms with his former club, who are understood to have offered £35m for his signature. Roberto De Zerbi confirmed on Sunday that Richarlison had made his desire to leave known, telling reporters: "Richarlison, I don't know. He said too many times he wants to leave."
Richarlison responded on Instagram in remarkable fashion on Monday, posting an emotional statement that placed the blame for the collapsed move squarely on others.
"After everything I've been through in the last few years, I've decided that never again will anyone decide my future for me," he wrote. "If they reach an agreement between Spurs and Everton, I will return to my beloved club.
"My intention was to go back to the place where I was very happy, yes, but the negotiations happened only in the way certain people wanted, and it will no longer be like that. Despite all this turmoil, I am under contract at Spurs and will remain at the club's disposal."
With the Tottenham route closed off, City moved decisively to secure Ndiaye instead, agreeing a package worth up to £65m — £60m guaranteed plus a further £5m in add-ons — to make the Senegal international Enzo Maresca's latest signing.
The deal for Ndiaye leaves City's separate £80m move for Liverpool forward Cody Gakpo close to collapsing.
Manchester City had opened talks over Gakpo on the understanding that Liverpool would only sanction a sale if they could bring in a replacement of their own, having already added Bradley Barcola from Paris St-Germain in a deal worth up to £123m on Monday.
Liverpool had targeted Brighton's Yankuba Minteh and Crystal Palace's Ismaila Sarr as potential incomings, but the valuations attached to both players proved problematic, and the Reds are now expected to keep Gakpo, 27, unless a fresh option for a winger can be found before Tuesday's deadline passes.
Ndiaye, who joined Everton from Marseille in 2024 having first made his name at Sheffield United, becomes City's latest attacking addition of a busy window. His departure leaves David Moyes needing to press on with alternative business in attack, with Everton already closing in on Monaco's Folarin Balogun as a separate signing.
For Richarlison, the statement brings a chaotic 48 hours to an end, though not necessarily a resolution. Should Spurs and Everton find common ground before Tuesday's cut-off, the door to a return to Merseyside remains open — but for now, as he made clear himself, that particular decision is out of his hands.