Foster, 27, is set to make his exit from Manchester United in a £6million transfer having failed to secure a regular first-team place at Old Trafford with Edwin van der Sar and Tomasz Kuszczak ahead of him in the pecking order.
Foster's fall from being United's number one keeper at the start of the season has also cost him a place in England's World Cup squad and Ferguson said he understood the keeper for wanting to seek first-team football elsewhere.
"We accepted the bid from Birmingham City simply because we knew how concerned Ben was about his future and I can understand that," Ferguson said in New York last night at a press conference to promote Manchester United's summer tour to North America.
"I think most of that comes from his disappointment of not having played enough games to get in the England squad this summer.
"I still feel very strongly about this, that Ben's the best England goalkeeper and I can't work out why he wasn't even considered simply because he didn't have enough football.
"Sometimes your conscience does come into it and you have to understand a player's feelings about it. Ben's been a terrific servant for us, he's been very unlucky with a couple of injuries he's had with us last season and I think he deserves the opportunity to do better for himself."
Ferguson said he would not be bringing in a replacement for Foster from outside the club but would instead promote England Under-21 player Ben Amos to the first-team squad alongside Van der Sar and Kuszczak.
"When we decided to let Ben (Foster) go, we were in the process of letting a good young goalkeeper, Ben Amos, go out on loan," Ferguson said.
"He's in the England Under-21 team but we can only accommodate three goalkeepers. So when the situation with Ben (Foster's) future at Manchester United came into it and we agreed to let him go, we stopped Ben Amos from going on loan.
"He will now be our third goalkeeper, he's very talented and has a good future ahead of him. "So we have three goalkeepers, Edwin van der Sar, Tomasz Kuszczak and Ben Amos. So we're okay."