Matthew Upson's agent has accused Birmingham of going back on their word in refusing to allow the defender to leave St Andrews.
The England international, who has 18 months left on his existing contract, has been the subject of several bids from West Ham, all of which have been spurned by Birmingham's board.
Blues boss Steve Bruce is loath to lose his defensive talisman and is thought to have turned down offers in the region of £7million, for a player who has repeatedly spoken of a desire to return to the Premiership.
Reports at the weekend have suggested West Ham are ready to return to the Midlands with a final £8million approach, with Upson's agent Barry Neville insistent that Birmingham have reneged on a verbal agreement to allow his client to leave.
Neville claims Birmingham told him that the 27-year-old could leave after four years, if they received an offer that matched a predetermined price, thought to be £6million.
"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," Neville told the Birmingham Sunday Mercury.
"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."