Auto Insert A Row With Info Populated From The Row Above

to be able to do is have a stock control sheet that tells me how much of 1 item we have in stock which is easy, the hard part comes when this stock is sold as this stock can be sold to a number of different customers e.g.

We have 10,000 X Pens

Customer 1 buys 1,000
Customer 2 buys 6,000
Customer 3 buys 3,000

i need a way of saying that we have 10,000 pens but if we sold 1,000 then we can click a button (macro maybe) and that will automatically see that we have 10,000 for that line and we have only sold 1,000 so we need a line inserting saying that we have 9,000 remaing and so on.... The sheet needs to record who these goes to though, that the reason i was thinking of adding a row

If this doesnt make sense then i am happy to answer your questions?

i can upload an example excel sheet if required if someone can explain how i do this?