Matthew Upson has accused Birmingham of "moving the goalposts" by snubbing a bid from West Ham, but the defender has vowed to give 100 per cent if he is not sold in January.
Upson has been one of the most talked about players during the January transfer window, as Blues have been forced to fend off interest from a number of clubs.
West Ham have firmed up their interest in recent days, but so far a £6million gambit has not persuaded the Blues to sell the centre-half.
Upson's agent said at the weekend that Birmingham had gone back on their word in refusing to allow the defender to leave St Andrews and Upson admits the events of recent days have left him frustrated.
"I think the fee that has been offered by West Ham is a realistic offer, a good sum of money, £6million plus bits and pieces, but we have a difference of opinion," said Upson.
"I will see how it goes over the next three or four days. It is a bit disappointing in that I feel the goalposts are being moved every week if you like.
"I know it has been labelled the silly season but I think Birmingham have had a pretty good offer for me and yet they have decided to decline it."
Upson will give full commitment to Birmingham for the rest of the season if he is not sold.
"If can't go, I'll give 100 per cent. I am paid to play football and I have my own reputation and I want to play well. I have a lot of respect for my team-mates, for the fans and myself.
"Whatever disagreement I have with the club, I have my team-mates and a stadium full of fans that I have to turn up and do my job for."