Software

Use a program to manage a transaction entry window, to both post new entries to the transaction record, and to decrement your stock list. It sounds like you want to maintain two worksheets, one a transaction register and the other a stock list, and have the simple act of entering a new row of transaction data perform an update on the inventory. I don't see how that is going to work.

Copy & paste the picking list and the stock list into different worksheets in the same workbook. If you're fully familiar with the distinction, I apologize for the previous italics, but many people are not, and the distinction usually needs to be emphasized, in my experience.

Now, you can use a vlookup function in the stock list worksheet to copy the quantity from the picklist worksheet, if and only if the code matches. The function in the stock list worksheet would look like =vlookup(picklist!$a1:$b65536,2,false), assuming that the code is in column a & the quantity is in column b of a worksheet called 'picklist'.

The next step is to use the result of that function, and subtract from the current number in stock. After that, the next step is to start using MS Access & treat these picking lists as link tables, with the functions described above replaced by queries.