A statement on the Cardiff website added: "We're unaware of whether he has been given permission to talk to Birmingham City, we only want players who are committed to Cardiff and have withdrawn from the deal.
"However disappointing this may be, we are completely focused on bringing in the right players prior to the new season starting."
Everton sold Bent to Charlton in January 2006 for �2.5m, with the Addicks being relegated from the Premiership at the end of the next season.
The following season Bent managed only two goals from 22 starts and 13 substitute appearances but was still wanted last season on loan by Wigan.
A better return of seven goals in 32 games helped keep Steve Bruce's side in the top flight but Bent became surplus to requirements at the Valley having returned from his spell at Wigan.
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