This gives you guidance on the Automatic Core Return process 

Allowing the system to generate the Core RETURN record on Purchase order on a stocking core item is very simple; the COR item is setup in inventory as shown below with the 'R' Popularity.  Refer to Core Default Line Code Setup to setup your CORE items with the appropriate defaults.  The example shown below is a Inventoried Core (core item setup in inventory) - the same process applies to Non Inventory core returns refer to Non-Inventoried Cores for instructions on the different prompts and settings involved to manage Non-Inventoried Cores.

When the sale with the customers Return Core is closed out, the system will update the inventory quantity and post a record in the Purchase Orders for the cores to Return to the Vendor for the credit.

Below is the Inventory Screen:

The Purchase Orders has the RETURN item as an Open PO item so it needs to be "ordered" when the Cores are sent back to the vendor and the Core RETURN records then need to be "received" when the vendor issues the core credit.
Below is an example of the cores waiting to be returned to the vendor for credit in Purchase Orders:

Below is an example of the same core being returned to the vendor and the vendor credit being received properly using Purchase Orders to track the physical item and post the payable credit from the vendor.

If the majority of your CORE returns will be handled automatically as was shown then setup your Default Core Line item with an R popularity. Below is an example of the Default line item: