Koren, who will become a free agent next summer, has reportedly begun talks over a new contract at the KC Stadium.
However, Bruce has revealed that the 31-year-old has a buy-out clause in his contract, understood to be in the region of £500,000.
"I've got to be honest and say things are complicated because he has a buy-out in his contract which I've found out in the last week or 10 days," Bruce told the Hull Daily Mail.
"We hope we can persuade him to stay. That's certainly our intention, but I'm not going to try and lie to supporters because there is a problem with the buy-out.
"If a club reaches that then we'll have a problem with it I would envisage. That's what we've inherited and that's what we've got to deal with. We don't want him to leave, but we're powerless really if someone matches that price and he wants to go. But I'll do my utmost to try and keep him."
Koren made 61 appearances for his country between 2003 and 2012.