Help with a ruling please.

[ukgazz]

Hi I would be interested if there are any official rules for this situation as I have seen it happen differently in various online sites and live games :

Blinds 200 / 400

SB Posts 200
BB can not post the full 400BB and is forced all in for 300

Do the players to act have to call the full 400 that the BB should be or can they call the lower 300 bet?

Thanks for the help.

UKGazz