Matthew Upson's agent has accused Birmingham of reneging on a verbal agreement with the central defender in blocking his move to West Ham.
Barry Neville claims the 27-year-old was told by Blues he could leave St Andrews after four years if any interested clubs met a certain price.
Neville says that figure has been exceeded by the Hammers, who have had a £6million offer rejected, but City still refuse to let Upson leave.
Upson's representative told the Birmingham Sunday Mercury: 'Matt is disappointed after having been told he could leave Birmingham for a certain amount - and what has been offered has gone way past that.
'There's not really much more I can say other than I'm disappointed and I'm disappointed for Matt.
'As to whether he would be interested in a move to West Ham, that can't be answered until a bid has been accepted.'
Upson rejected a new five year contract last season and has made it clear he will not put pen to paper on another deal for Steve Bruce's side.
He has 17 months remaining of his contract and Bruce on Friday slapped a 'not for sale' tag on Upson as regards this transfer window which closes at midnight on Wednesday.