fair or foul ???

[bobbrix]

here's a weird one from a fast pitch debate today

- batter swings and fouls the ball back
- ball goes up at angle and hits one of the metal support
posts of the BACKSTOP
- ball bounces back, hits ground just inside 1st base
foul line
- ball continues to roll and ends up stopping just inside
the the 1st base line on fair ground

fair or foul ???

i know the backstop is considered to be part of the ground,
but is it ok to bounce off it and end up with a fair ball??

(i called it foul)