New lease of life: Foster will move to St Andrew's for £6m
The two clubs held talks over the weekend and have struck a deal for the 27-year-old with a basic £4million being paid up front and the rest added according to performances and international caps.
It ends a period of frustration for Foster who has struggled in the shadow of Edwin van der Sar and realised his days were numbered with United scouting Manuel Neuer at Schalke and, number one target, Hugo Lloris of Lyon in recent weeks.
Foster said his goodbyes to teammates at Carrington on Saturday and will look forward to regular football at Birmingham from next season.
Birmingham also hope to sign Rangers striker Kris Boyd over the next few weeks on a free transfer.
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