Birmingham have rejected a bid of £5.5million, plus central defender Bobo Balde, for striker James McFadden from Celtic.
City chairman David Gold has insisted Scotland international McFadden is 'not for sale'.
Celtic fan McFadden pledged his future to Blues when it became clear their relegation from the Barclays Premier League was inevitable.
And Birmingham manager Alex McLeish knows keeping the likes of McFadden and Sebastian Larsson is crucial if they are to mount a strong promotion challenge.
The Bhoys were interested in taking McFadden on loan from Everton during the January transfer window.
But McLeish's £5million offer plus add-ons was accepted by the Toffees.
He was already well versed in McFadden's capabilities from his spell as Scotland boss and he impressed for Blues before suffering a knee injury.
Gold told PA Sport: 'James McFadden is not for sale. We want to keep our best players to help us try to get back to the Premier League as quickly as possible.'
Meanwhile City have failed in their bid to sign veteran Czech Republic striker Jan Koller, who instead looks set to join Russian Premier League club Krylia Sovetov Samara.
Blues had agreed an undisclosed fee for the 35-year-old with FC Nuremberg and were ready to offer Koller a two-year deal.
But his country's record goalscorer has received a more lucrative offer from Krylia.