Interesting
I don't think so you can manage this From Location Code and Special Order.
Furthermore, you can't manage it from any change in Costing Method.
What system is doing doing is totally Logical. It will check the Quantity on the basis of the Sales Order Quantity you Wrote.
If you want to see that Out-of-Stock by 950 or any unique thing, then it requires Customization.
In Standard Process this is totally Logical.