England international Foster has seen his chances limited at United this term, which has cost him a place at the World Cup finals. His agent Lee Payne has confirmed to skysports.com that his client is in talks with Birmingham.
"We are close to agreeing a deal," he told skysports.com.
"The two clubs have agreed a fee and given us permission to talk to Birmingham.
"We fully expect the deal to be completed either this weekend or the start of next week."