This has already been discussed & answered (read all posts in that topic) in the link to @Lubos topic in your first post of this topic, i.e. Added "Qty available", "Qty to be available" and "Qty to order" columns to “Inventory Items”. We also discussed it here so for the final time also read post number 6 at Help with recent changes - #6 by eko
For all clarity, your steps are incorrect. The basis is the Sales Order not the Sales Invoice. So once again:
- Customer sends their order.
- We issue a ‘Sales Order’.
- Customer agrees and pays.
- We copy the ‘Sales Order’ to a ‘Delivery Note’, and also;
- We copy the ‘Sales Order’ to a 'Sales Invoice", and also;
- We copy the ‘Sales Invoice" to a "Receipt’