The 20-year-old is of the brightest talents in Europe and Dortmund, who finished second in the Bundesliga last season, had slapped a 120 million euros ($140 million) price tag on him.
Media in the United Kingdom reported last week that United were close to signing the starlet.
Reports of a switch to United became rife over the weekend after Sancho was spotted at a party in London before returning to Dortmund.
In an interview last week, Dortmund CEO Hans-Joachim Watzke ruled out a transfer of Sancho once their week-long camp in Switzerland started Monday.
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