The fee is £2m more than when Harry Redknapp last sold the player, whilst managing at Portsmouth.
And Saints boss Redknapp added that he had been thoroughly impressed with the attitude of the England international, despite the ongoing speculation regarding his future.
"He's got (a) terrific touch," Redknapp said. "He's good in the air, has a good understanding of the game and is a fantastic player. He's a good lad and has not been a minute's problem even when this has been going on," he added.
"He's come in every day, worked hard and been a fantastic example to everybody. What's special about him is he's 6ft 8ins. That's probably the most special thing. I wish him well and I'm sure he'll be a big success at Liverpool," Redknapp concluded.