Doyen own 75 per cent of Rojo's rights and Sporting are contractually bound to pay the fund 75% of any rejected bid. United have so far tabled an offer worth a reported €20 million, which falls short of his €36m release clause, meaning Sporting will have to pay €15m if they continue to reject the Premier League club's advances.
Doyen have confirmed that they will vigorously pursue that figure, whilst also threatening the Portuguese outfit with legal action if no payment is made.
“(Sporting are) making mindless demands as a condition of releasing the player,” they said in a statement.
“If Sporting want to keep Rojo they must compensate the fund under the terms and deadlines set forth by the contract.
"If necessary, we will not hesitate to use every legal resource at our disposal to fully defend our rights and interests.”
Sporting have since cancelled their contract with Doyen, announcing they have terminated the agreement with “just cause".
Rojo has also been linked with Southampton this summer but Louis van Gaal appears to be in pole position to land the Argentina international as he seeks to further reinforce his defence ahead of the 2014-15 season.