The 22-year-old has been linked to numerous clubs, including Liverpool, Arsenal, Crystal Palace and former club Tottenham Hotspur.
Widespread reports have claimed that the defender has an £8m release clause in his contract, which can be activated following the Bluebirds' drop into the Championship at the end of last season.
"We've agreed a fee with Cardiff and it's now up to Steven to decide his next move," The Telegraph quotes Redknapp as saying.
"I know him well from Spurs and he's always been a tremendous prospect. Hopefully that's another one we can get done."