The agent of Henrikh Mkhitaryan said it would be wrong to abandon attempts to get the Borussia Dortmund midfielder a move to Manchester United.
The Bundesliga club are refusing to let the 27-year-old leave following the departures of both Mats Hummels and Ilkay Gundongan this summer.
"It would be a sin to stop talking," said agent Mino Raiola.
"I am a positive guy. If I wasn't confident of reaching some kind of conclusion, I wouldn't try."
The Armenian has a year left on his contract and has told Dortmund he will not sign another deal, with United his preferred option.
Raiola added: "I am very proud Manchester United want him and the talks are being held in a good atmosphere, but at the moment Dortmund are still saying no."
New United manager Jose Mourinho is attempting to improve his squad after replacing Louis van Gaal.
He has already signed Eric Bailly from Villarreal, subject to the Ivorian being granted a work permit.
United are also expected to sign Zlatan Ibrahimovic once the Swede is free of his international commitments at Euro 2016.
Ibrahimovic, 34, is a free agent after leaving Paris St-Germain but Raiola, who is also the agent for the Swede, says he is still to decide his future.
"Manchester United is a great club, with a great coach," said Raiola.
"But Zlatan has not yet made his choice."
Ibrahimovic has had lucrative offers from China, the United States and Europe.
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