Validation Rule

Hey there,

I have an inventory document that contain various sheets. Each sheet represents a card of an item.
One of the fileds on each sheet as Current Inventory of that item.

On that sheet I made a form where the user can indicate how many of that item s/he has taken.
a validation rule is set to guard that the user cannot take more than what is available.

For example:
G8 = current invetory amount
B12...Bn = are the cells where a user will indicate an amount taken

G8= 23,6 (initially)
B12=23 (G8 is now 0,6)
B13 = 0,3 (G8= now 0,3)
B14 = 12 (error message should appear)

I used this formula in the validation rule window: =$G$8-$B12 >=0
But it does not work well

How can I solve this best way?
Thank you,

When I test this again, even when I enter in B12 23 I get an error message - so the formula I use is not reliable