Mauricio Pochettino's side have first refusal on the player as part of the deal that took the Welshman to the Spanish capital in the first place, meaning that were they to match United's offer for the 28-year-old Real would not be able to say no.
Daniel Levy is well known for being a difficult man to negotiate with and it seems he pulled of another transfer masterstroke with the sale of Gareth Bale.
As well as tempting a then world-record fee out of Real Madrid for the player, he also negotiated a first refusal clause, in the event the player ever wanted to return to England.
With Manchester United now reportedly ready to do everything in their power to sign the player, 'The Mirror' points out that any move to Old Trafford would be a complicated affair.
The clause means that should Real Madrid opt to sell Bale, Spurs can effectively set the player's price as they can force interested parties to pay over the odds by threatening to use their first refusal below a certain price.
Another option would of course be that Spurs simply use the clause to bring the player back to the club that made him the global superstar he is today.
At the moment though, Real have made no noises that they are prepared to sell the player but should they do so, Mourinho may well have to fight for the Welshman every bit as hard as he said he would in a recent press conference.