Kinsey
   A function or formula is not going to do what you want, you will need
VBA programming.  To get help with this you need to furnish some information
about the layout of your data, both the order form and the inventory sheet.
For the order form, specify the range that will hold the item names or part
numbers or whatever you use to identify an item.  Do the same for the
quantities.
In the Master Inventory, do the same for the ID and quantities ranges there.
Finally, you will need a trigger (something you do) that will tell Excel
that the order form is complete and is a valid order form for Excel to act
on.  Perhaps the action of printing the order form, or maybe you would
prefer to have a button next to the order form that you can click on when
the order form is complete.
HTH  Otto