Foster has made a £6million switch to St Andrew's in a bid to play regular first-team football after becoming the third-choice keeper at Old Trafford.
His move down the pecking order ended his hopes of being part of England's squad for the 2010 World Cup finals.
But Foster has nothing but admiration for Ferguson despite starting only two first-team games since playing back-to-back internationals against Belarus and Brazil last autumn.
Foster said: "Had my relationship with Sir Alex become strained by the end? No, definitely not.
"I've read quite a few things saying that we had a massive barney after the Sunderland game. A barney involves two people talking. It was mainly just one person!
"But the relationship was definitely not strained. Sir Alex has been fantastic with me and I've read some nice things he said the other day which is fantastic to hear.
"It gives you a boost. You never have to say anything when a player leaves his club. I got on with the manager really well up until the end, which unfortunately had to come."
Foster concedes he had to leave United to further his career.
He said: "I've been at United five years, two of them on loan at Watford. I've really enjoyed my time there. I've been out on loan for a few different teams and seen that side of football.
"To be at Manchester United is a different world but the last couple of years have been at up and down for me, in and out of the team.
"You come to a point in your career when you realise it's not going for you at a certain club and that time has come.
"I'm delighted to join Birmingham. It's a team with ambition, a team looking to continue on from last season's progress and I'm just happy to be here.
"I know I was at Manchester United, the biggest club in the world, but I don't enjoy not playing regular first-team football.
"Training is all good but you've got to have something to look forward to and that is what is on offer now."