I personally think that there is surely a possibility of a clause. Because as someone said in the januzaj deal, you have an incentive to play him only if you gain something out of it. Else he will end up a squad player there too. With a clause they may be encouraged to play him often which will benefit him too.
At the end of the year, we can always buy out the purchase option if we want. I think Adnan would prefer United over Dortmund as things stand and like alderwereld was sold to Spurs even though Southampton had a buy out clause, it's really not a compulsory sale.
