The 20-year-old enjoyed a successful seven-month loan spell at the KC Stadium last season, scoring 15 league goals.
Duffen told BBC Humberside: "We've put a £7million bid on the table to Manchester United.
"The complexities of their situation are that it depends on them getting one in, that one might well be Bulgarian (Berbatov).
"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.
"And we are going to have to get braver in some of the things we do but it will never be at the cost of threatening the security and financial ability of the club long term."