Continuous Count

Hi

I have a table like below where I have to determine the Available Quantity based on the running count of  Qty On Hand and the Order Received Qty. If the order received is greater than the Avail QTY to sell then by default it should show 0.

Initialy Qty On Hand was 5 and Order received Qty was 2 so the Aail QTY To sell was 3

Now for the next order Qty On Hand is  3 not 5 and Order Received Qty is 1 so I can fullfill the order.

Now the Avail Qty To Sell is 2 so is the Qty On Hand but the Order received Qty is 3 which is greater than the Avail QTY to sell so I cannot fullfill the order hence it should show the Avail Qty To Sell 0.

Thanks