Physical negative inventory and financial negative inventory - cannot help you in without having the physical cost or financial cost.
It helps you in consumption or sale of goods with out stock and stock value.
If you don’t want to track the cost - put zero for the price during the transaction - it really does not matter what ever the Inventory model may be if you are not tracking the costs…
In the case of return sales order - how the sales order was posted? - with price or with out price.