Bruno Fernandes has emerged as one of Manchester United's top targets this summer but the Red Devils are yet to make a firm bid.
Fernandes' agent Miguel Pinho is reportedly in the process of agreeing personal terms with Manchester United ahead of the Old Trafford club making an official approach.
United are reportedly ready to hand the Portuguese a contract between £4.5million and £5.3million-a-season in wages, which has 'seduced' the 24-year-old.
But Sporting Lisbon and Manchester United remain someway off agreeing on a transfer fee, with the Red Devils wanting to pay around £32million for the midfielder and the Portuguese club holding out for a much higher fee.
Sporting would reportedly be prepared to sell Fernandes for around £53million, despite the fact he has an eye-watering £90million contract release clause.
But a 'side deal' could give the Red Devils a considerable advantage where negotiations are concerned.
According to Duncan Castles, who spoke on the Transfer Window podcast this week, Sporting made a gentleman's agreement with Fernandes last summer.
At the time the Portuguese club was in disarray with players ripping up their contracts and simply walking away.
In an attempt to keep Fernandes for at least another season, Sporting reportedly made a 'side deal' with the midfielder that said he could leave next summer if a £32million bid was made.
And if the club decided to reject the bid they would have to pay Fernandes £5million compensation.
This 'side deal' means that in theory United could sign the midfielder for the price they want to pay - £55million less than Fernandes' release clause - should Sporting want to avoid paying the player £5million.
