Birmingham managing director Karen Brady claims she felt 'mugged' after losing Matthew Upson to West Ham.
Former England centre-back Upson, 27, joined the Hammers on transfer deadline day in a deal that could be worth up to £7.5million.
It brought to an end a protracted affair which eventually saw the former Arsenal defender demand a move after several lower bids were rejected by the Coca-Cola Championship club.
Upson's sale was met with dismay by City manager Steve Bruce - who has since reaffirmed his commitment to the club - and Brady has insisted cashing in on the player was the last thing on the board's mind.
'The truth is I feel as if we have been mugged,' Brady claimed in her column in The Sun.
'I'm the money person at St Andrew's and my own frustration at the conduct that cost us our England international centre-back is intensified by what we stand to lose.
'With Upson in the team the manager believed we had a better chance of promotion to the Premier League than without him.'
Brady added: 'We knew it was serious when Upson actually phoned (City co-owner) David Sullivan on holiday in Barbados and told him of his desire to leave.
'We tried offering him a compromise, stay with the club to the end of the season and if you still want to go we'll sell you - even though we knew it could mean getting less than half that was on the table now.
'But nothing other than capitulation was good enough. Upson made it clear that he saw his future away from us, stating that even if we got promoted he would not consider signing a new contract at Birmingham.
'It wasn't even as if West Ham were a major club. Had Chelsea pursued their interest in the player, I think most of us here would have said `goodbye' and wished him well. That went for Jermaine Pennant when he left last summer for Liverpool.
'But the Hammers aren't top four material or anything like it.'