ESPN says United have just one week left to sort any move though are believed to not be willing to pay the £108m asking price.
United are willing to agree a structured deal that would include a series of performance-related bonuses.
Though it's added that Dortmund would prefer the bulk of the fee up front and have concerns about add-ons.
One relating to participation in the Champions League is reportedly a particular concern with United having missed out on a place in the competition altogether three times since 2013.
It's claimed United are pushing to pay £65m up front, with a further £25m due in bonuses over the length of Sancho's contract.