If this part of the Guardian article is true, then it sounds like Sporting is trying to cheat Doyen out of the transfer fee. According to wiki, Sporting paid £3.5m to Spartak Moscow for the transfer, and Doyen would have paid the remaining £10.5m for a total of £14m for the transfer. Now if United offered £16m then Sporting would receive only £4m which would recoup what they paid, but they won't be making anything. I guess they want a bit more compensation having had the player for two years but the reality is that they wouldn't have gotten him in the first place if Doyen hadn't helped pay his fee, they wouldn't have afforded him at all.