I have a few questions that need answering before a satisfactory solution can be offered. The reason for the question is also given to give you an idea why the question is asked.
1. Are more then 1 copy of the stock sheet going to be in use simultainiously in diferent locations? If there are more than 1 concurrent user, each with their own sheet open, then when each user clicks to deduct a quantity from the stock, it will have to query the stock item quantity to see if it is greater than zero, (in case someone has already allocated the stock in their open sheet) as it could create a minus stock value.
2. How are you going to control stock receipts? If you get stock in, at what stage is the stock going to be incremented, without corruption of the totals on open sheets. (Again query before deduction is indicated.)
3. Why not use Access instead of Excel? Access is a far better medium for this sort of thing. There are simple stock management databases already in the public domain, and it is not as memory hungry as Excel.
An Access db form can be created to do the stock deductions, and reading between the line you might later want to record who sold it, at what cost, and to which customer for commission purposes, or audit purposesetc.