Validating
 

I have a spreadsheet that tracks Inventory. I use a pull sheet for the user
to enter the product and the amount to be pulled. Once this is done by using
a formula the master inventory sheet is automatically updated. I want excel
to prompt the user if the on hand inventory becomes less than 0 based on the
amount they want to pull out of inventory. Example:

Pull Sheet:
Date Prod Id Customer P.O # or Job Name Need Pulled
BM3070 1 1

Inventory Sheet:
Prod ID Size Finish Price On Hand Rating Species
BM3070 3/0 x 7/0 Mahogany $131.55 -1 Non Rated Birch
M3070 3/0 x 7/0 Unfinished $194.00 10 Non Rated Mahogany

in this case I pulled a product not on hand which shows -1, when the user
goes to the master inventory I want a message box to indicate all products
that may be less than 0