How to configure Back to back Transfer and Buy flows together?

Summary:

We are trying to configure the Back-to-Back Transfer and Buy flows to work together for the same items, as this is the standard business cycle:

The objective is to :

Check the quantity in the main shipping org entered in SO, IF NO qty Then :

1- Check if there is a sufficient quantity in the "transfer from" organizations to fulfill the SO, If No Qty Else,

2- Create a Purchase requisition to buy the requested Qty.

The configuration created:

1- A global sourcing rule :

2- Assignment Set on org level

3- ATP Rule assigned to org

Result:

SO Order fulfillment: create a transfer order from the other org, even if there is no qty in both org, and the buy flow does not work