Create a running total {#orders} using the running total editor. Select {table.orderqty}, sum, evaluate on each record, reset never. Then create a formula {@available}:
{table.inventory} - {#orders}
You could lay this out like:
{table.inventory} {table.orderqty} {@available}
10,000 9,000 1,000
500 500
Suppress {table.inventory} using "Not Onfirstrecord" in the field suppress formula area.
Or if the issue is that you want the layout you show above, then just create three detail sections, add the labels on the left (again suppressing the inventory label using "not onfirstrecord" and the fields next to them to get:
detail_a: Inventory: 10,000
detail_b: Order Qty: 9,000
detail_c: Available: 1,000
detail_a:
detail_b: Order Qty: 500
detail_c: Available: 500
-LB