Auto Update Stock Quantities

I have a list of sales items (database query) that are due over the next 12 months all sorted in date order (approx 300 lines) I would like to add a column showing the available stock for each line taking into account earlier sales items quantities.

I have attached an example spread sheet showing the sales items (A:D) and the stock quantities (I:J)

For part number 456DEF I have shown in green the result I am looking for however what would the formula be to automatically calculate this down the 300 lines?

The Sales Items & Stock quantities are both taken from a database query that refreshes every 20 minutes.

Thanks in advance
Dougie