Birmingham's managing director Karren Brady says she felt "mugged" by Matthew Upson's departure to West Ham.
Centre-back Upson made the move to The Hammers on transfer deadline day in a deal that could eventually be worth up to £7.5million.
The 27-year-old demanded a move after several lower bids were rejected by Blues with boss Steve Bruce dismayed by Upson's departure.
"The truth is I feel as if we have been mugged," Brady said in her column in The Sun.
"I'm the money person at St Andrews 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.
"We knew it was serious when Upson actually phoned David Sullivan (City co-owner) 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."