Even though Dortmund's sports director Sebastien Kehl has reaffirmed that Sancho will be going nowhere during the transfer window, United are reportedly still trying to secure the 20-year-old's signature.
Dortmund were believed to have set an August 10 deadline for a deal to be completed, and the Bundesliga outfit are supposedly scratching their heads over United's approach to the situation.
Transfer expert Dharmesh Sheth told Sky Sports News: "Time is running out for Manchester United to make an official move, and even if they do, Borussia Dortmund are simply not budging. I'm told today that United knew well in advance of an August 1 deadline to come to the table with regards to Jadon Sancho.
"That was extended to August 10 but United hadn't made any contact before that date. Instead, they focused on perceived issues surrounding agents fees and personal terms. Dortmund have consistently said that Sancho will play for them in the coming season.
"As recently as Monday, the Dortmund director Sebastian Kehl said Sancho would be staying at the club. He said, 'it is an important statement because we have a certain responsibility and without Jadon, this team is worse'.
"He went on to say, 'yes, some day he may well leave for a new challenge, but right now he's happy at Dortmund and Dortmund are happy to have him'. I'm told today that Dortmund and those close to the deal have been 'bemused' at how United have approached this deal."
Man United were said to be weighing up a move for Barcelona forward Ousmane Dembele as a potential alternative to Sancho, but a recent report claimed that Barca are not willing to sanction a sale for the Frenchman, forcing United to continue their search for a new attacker elsewhere.