Morata, 24, has emerged as Jose Mourinho's top target to bolster the United attack ahead of the upcoming season since Antoine Griezmann chose to continue one more season at Atletico Madrid. Meanwhile, the chief was also coy over the future of James Rodriguez following recent reports claiming that the Colombian global is also set to move to Old Trafford.
Morata, 24, had hoped a deal would be well down the road to completion by now, but with Real holding out for at least £75 million after rebuffing an offer of around £65 million from United, negotiations threaten to drag on.
Other reports have claimed that United are now offering €74m for Morata - in spite of what Perez is insisting on - but Los Blancos are confident of getting between €85m and €90m if they are to sell him.
The Real star has been linked with a move to the Premier League, with United tipped to sign Rodriguez.
"There will be time, we will do what is best, but Madrid is not a seller".
Morata is already thought to have agreed personal terms ahead of a move to Old Trafford, but Jose Mourinho's side are understandably reluctant to part with such a high fee.
"He is one of the best players in the world, he belongs to Madrid and Madrid players do not leave when they want to or their parents wants them to", he declared.
Perez said the back-to-back Champions League winners had considered signing the France worldwide, but decided he was not needed in their first XI. "And above all prevailing the interests of the club".