The Mirror says United were tipped to pay just £135,000 for 16 year-old Mejia's training rights with the youngster reported to have said goodbye to his teammates.
But Real Zaragoza have now hit back and insisted they know nothing about the reported agreement.
The La Liga club released an official statement denying a move to Old Trafford had been agreed.
It said: "Given the information in the United Kingdom about the Real Zaragoza cadet footballer Mateo Mejía and the interest of Manchester United for his signing, Real Zaragoza wants to clarify the following:
"Mateo Mejía is a footballer who belongs to the discipline of Real Zaragoza in the Cadet (academy) category, linked to the Club with a federative card until June 30, 2020.
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