The Dutchman has given his approval to the move.
"I am currently negotiating with Besiktas," said agent Aleksander Bursac, who recently stressed United did not want to allow the Dutchman to leave.
"Büttner has wanted to join Besiktas from the beginning. He has been watching the Turkish league and Besiktas closely, was really impressed by the supporters and knows the club's history."
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