Stock list macro

Hi. Im creating a macro for a whole ordering database that includes an order form, invoice and a stock list. I have done a few macros for the order form and invoice which basically copy and paste data into differant places and then deletes them when the user requires.
However i have now come onto the stock list and cannot find a way of updating the stock at the click of a button. Iv used drop down boxes for product selection on a seperate sheet and then to the right of this a Quantity demanded column. I want that quantity for that product to be taken away from a stock list on a seperate sheet.

This may sound very hard to understand but if you could see the database you would see its very simple (apart from this bit )

So it is like this
3 sheets:
1) order form
2) invoice
3) Stock list

1) in the order form there is an order sheet which has drop down boxes of lists of products that the shop sells which are taken from a big product list above which has prices to the right. Vlookups have been used to correspond those prices to change when a selection is made with the drop down box. To the right of this is a quantity demanded column where a number is entered. I then want that number to be taken off the corresponding product in the stock list which is just a duplicate of the huge product list from the order form.

I hope you can help me that probably all makes no sense lol but o well xD thanks for any comments/help

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