Don Balon says there were two reasons for United's decision.
First, Barca's insistence on a buy-back clause for a set price was rejected by United. There was the sense at the English club's end that Barca could easily re-sign Mina and sell him for a profit without United having any influence on the matter.
Secondly, Barca were insisting they also take Andre Gomes in a package deal. But United were only interested in the centre-half.
In the end, Everton signed both players hours before the Premier League transfer window closed.