Formula for Running Balance of stock levels

Hi,

I have a spreadsheet which lists customer orders and the product, the qty on order, and the current stock on hand we have. I would like to have a formula that will calculate a running balance for each product, so I can see which orders can use the stock and which can't. I have attached a spreadsheet, where column H is where I would like the running bal (highlighted yellow). At the moment I have manually entered the value so you can see what I mean.

Thanks!

Linda