Cardiff will allow Roger Johnson to leave the club for Â£5million if the terms of an offer are satisfactory, according to chairman Peter Ridsdale.
Birmingham have already seen two bids rejected for the 26-year-old, the second of which was valued at Â£5m but included payments spread out over a number of years and dependent on the club's top-flight status.
It is widely expected Blues boss Alex McLeish will come back in with an improved offer for Johnson this week and Ridsdale has admitted the same bid but with more favourable terms will see them part ways with the centre-back, who has revealed his desire to have a crack at the Premier League.
Ridsdale told BBC Sport: "A fee of Â£5m, subject to how it's paid, is a figure where we'd have... to put it to Roger.
"I've had informal feedback from Roger that he would like to talk to a Premier League side. Clearly we wouldn't stand in any player's way if the price was right. Regrettably every player has his price but we're not letting people go on the cheap."
He continued: "Birmingham have made their interest clear and the amount they're prepared to pay. I've given them a view of our minimum price for Roger and as we speak Birmingham haven't been able to match the price or the payment terms. If Roger was to leave, clearly we'd have to move swiftly to replace him."
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