The Sunday Mirror says Mata had a verbal agreement with Blues boss Jose Mourinho that if he wasn't a first-team regular by Christmas, then the London club would let him go during the January transfer window.
Before Christmas, Chelsea put in writing permission for Mata's representatives to broker the Spain midfielder's transfer.
But he was stunned to discover in this document that the exit strategy came with the caveat that he could move to any club willing to meet Chelsea's £37m valuation - except United.
However, they had to soften their hardline stance when it became clear that United were the only club willing to pay £37m for the Spanish star.
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