That is the shock revelation from Arsenal chief Dick Law, speaking exclusively to Goal.
Back in 2017, Sanchez was the big transfer saga of the summer as Manchester City chased his signature.
Pep Guardiola was keen on signing up the Chilean – who he worked with during his time as Barcelona manager.
With one year left on his contract, City believed they could pick him up in the summer and a move was made on deadline day.
But Arsenal could not get a deal done with Thomas Lemar, then of Monaco, so the transfer never materialised.
Fast forward five months and Sanchez was in the news again, this time with United – where he penned a £500,000-a-week contract.
His time at Old Trafford has turned quickly into a disaster and former transfer and contract negotiator Dick Law has revealed the truth behind the situation.
“We had a deal,” he told Goal.
“It was in December and we had been talking to his agent [Fernando Felicevich] throughout all of 2016, the whole year.
“We got to December, I flew to Santiago and we did the deal.
“The player agreed to it on a phone call from Fernando’s office. Fernando and I shook hands and discussed how difficult the negotiation had been and we had the deal done.
“The contract team drafted the new contract and sent it over to the lawyers representing Sanchez and in that space of time, while the documents were being proofed, he changed his mind for whatever reason.
“We don’t know who got to the player, or why he changed his mind. But the bottom line was, he did.”
Sanchez has not enjoyed his time at United, with poor form and injuries hampering his career and reports have emerged that the Red Devils are trying to offload him.
“I don’t think Manchester United have got what they paid for with Sanchez,” admitted Law.
“United drive a hard bargain for players, they don’t usually go crazy with wages – but for some reason they went crazy on Sanchez.
“We never understood the transaction and I think Sanchez was just pushing to get out the door, but in retrospect I think he miscalculated.
“Is he a richer man today than if he kept his word on our offer in December, 2016?
“Yes, he has earned a few more millions. But I don’t know that he is more satisfied and I don’t know where he goes from here.”