This give you guidance on the Manual Core Return process:

Manually managing Core RETURNS to Vendors is suggested if you have a good process in place for handling cores, if you are a re builder (rebuilding your own cores) or if you do not have the Purchase Order system.  With this setup the Cores received from customers will simply go to inventory and you will manage them by issuing a RETURN (negative quantity PO) or adjusting the quantities manually (and updating accounting with credit information).  Inventoried Cores (cores setup in inventory)  would need to be setup with the "N" 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 to show the cores now on hand.  
Below is the Inventory screen:

To manually return the Cores the user will need to either check reports to see Core's in stock to manage the COR items or use the Inventory Search to locate the COR items in inventory with positive quantities.  Note:  COR items should never show negative quantities as they are not directly sold to the customer - remember that the Core transactions are all triggered by the sale of an item with a core value attached.

 (if using Purchase Orders) To send the core back to the vendor for credit the user will need to use the Order Ctrl/O option on the Inventory item and order a negative amount to reduce the on hand quantity as shown. see examples down further for instructions if NOT using Purchase Orders...

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 not using Purchase Orders) The Core items quantities will continue to build and be overstated in your Inventory Reports if the quantities are not relieved, if you are not using Purchase Orders you will have to do this manually by adjusting the quantities; there are several ways to adjust quantities on of the easiest is to use the UQty button available on the Inventory Search screen, when prompted enter a negative quantity and a note that either always the same or notes the vendor credit memo.

If the majority of your CORE returns will be handled manually as was shown then setup your Default Core Line item with an N popularity.