Counter formula that subtracts with each transaction
I do appreciate the effort here to help me with creating a running inventory counter. I realized at some point I better take the initiative and delve into some script so as to create an 'action' on command customized to my needs.

The idea was to fill entries of a Packing Slip with item numbers and quantities of that item sold. Then on another sheet that kept track of inventory, the quantity of current items sold would deduct from the remaining inventory. So a script was needed. (a formula would otherwise always revert back with each new entry.)

Basically, on the Inventory sheet (which had columns of item numbers and starting inventory) I used an IfError function with Match and Index nestled to find and transfer the corresponding entries from the Packing Slip. From there it was a simple process of scripting to create a counter.

Probably preaching to the choir here, but if anyone is interested, I can furnish the procedure, which involved a combination Numbers functions and applescript.

I also got a little fancier and insert a chime sound on completion as a reminder that the current order deductions were made from inventory, along with a notification banner.