Now, the details of the proposed transfer have been revealed for the first time, with Football Leaks publishing the contract that De Gea would have enjoyed in Spain had the move gone through.
It reveals that the goalkeeper would have been paid €11.8million-a-year - around £9.1million - plus incentives until June 30, 2021.
De Gea would also have received a one-off bonus of €10.9million (£8.4million) as a signing-on fee, to be paid in January 2016, together with an additional monthly salary of €6,000 a month.
In total, De Gea would have earned €86,138,183million (£66.7million) over the course of six years, had the deal been completed.
De Gea would also have ceded 50 per cent of his image rights to Real, and would have had to have paid the club €500million (£387million) a year each year if he had pulled out of the contract.
The document - which is entirely in Spanish - also confirms that Real had submitted registration documents for the player with the Spanish Football Federation.
At the time, United insisted that they had behaved professionally with regards to the transfer.
In a statement, United said: “The fact that Manchester United filed the papers on time was acknowledged by the Football Association, who offered to support that claim in any discussions with Fifa.
“The club offered this assistance, as well as its own time-stamped documents, to Real Madrid but they have chosen not to go down this route.
“Manchester United acts appropriately and efficiently in its transfer dealings.”
De Gea subsequently signed a new four-year contract with United, although Real retain a strong interest in the player.
Football Leaks has developed a reputation for making life highly uncomfortable for club executives, pursuing an agenda of introducing more transparency to football.
They first made waves by publishing the details of Gareth Bale's world-record transfer from Tottenham to Real Madrid, which prompted panic among Bernabeu executives, anxious not to upset their star player - and previous world record transfer-holder - Cristiano Ronaldo.
- Telegraph