Birmingham had already broken the League One incoming transfer record twice in this transfer window before completing the deal to sign Stansfield.
He scored 13 goals in 37 appearances during his spell at St Andrew’s, which saw him recognised with the club’s player-of-the-season award.
Stansfield scored against Birmingham in midweek in Fulham’s 2-0 win in the Carabao Cup second round.
Sky Sports News understands it is an initial £15m, but sources in Birmingham say it is less.
Christoph Klarer and Willum Thor Willumsson were both signed for fees in the region of £3.5m, but the Stansfield transfer smashes both of those numbers.
