Multiple Cells, Multiple Spreadsheets

[MartinMuster]

Hello everyone,

I would like to have some help regarding an in my opinion pretty difficult problem.

cell spreadsheet1 B3 + cell spreadsheet2 C4
cell spreadsheet1 B4 + cell spreadsheet3 D4
cell spreadsheet1 B5 - cell spreadsheet2 C4
cell spreadsheet1 B6 - cell spreadsheet3 D4
(these are just random values)

I will try to explain this in more detail.
I have multiple spreadsheets and I need the formula to take the numeric value of the spreadsheet outgoing or incoming, 'look at' a cell in the same row and based on that add or subtract this numeric value from the stock which is on another spreadsheet.
Each of those sections of the stock are in different spreadsheets.

I have uploaded a file and added some sample values.
Let's say I buy 3 pencils and I give out 2. It's supposed to say 11 in the spreadsheet writing utensils at the column stock.
And let's say I buy 2 scissors and give out 4. It's supposed to say 7 in the column stock of the spreadsheet misc.

I hope I explained my problem well enough for somebody to help me.
I'm not sure if array formulas are enough, if VBA or something similarly complicated is necessary, I'd like to hear that solution to this problem as well.
Your help is greatly appreciated.

Kind regards

Martin