United are pushing to close a deal for the Colombia international, who has already struck personal terms with the Premier League giants.
"Do you take us for fools?" was Mourinho's reply to the intermediary who relayed Perez's proposal.
Mourinho wants to sign James independent of any negotiations over De Gea - and certainly will not consider a straight swap.
The United manager rates his keeper at €70 million, while he believes a price of €40-50 million is a realistic valuation for James.