"We've put a £7million bid on the table to Manchester United," Duffen has told BBC Radio Humberside.
"The complexities of their situation are that it depends on them getting one in, that one might well be Bulgarian.
"As I said all the way through the summer, we need to be respectful of that and there's nothing I can do to force the situation.
"If they do bring someone in I want to be at the front of the queue and I will do whatever it takes to bring Campbell to the football club.
"There will be a lot of people alluding to the fact we've got carried away with ourselves.
"But I can absolutely guarantee that I'm here for the long term, building the football club."