Using Lookups to change a value in the lookup table

Hey all,
I have a table which contains information about stock, item description, price, barcode, stock qty etc.

I have used lookups to make an invoice so I can sell the product and print a receipt etc. I want to be able to take away the amount sold off the specific product that was found using the barcode and a lookup and take that amount from the stock qty column in the lookup table.

Is there a way in which this can be done? If so how?

I have included an example of the invoice and the lookup table that I want the stock to be taken from.

I have little knowledge in excel and any help would be greatly appreciated.

Thanks

Example files