Assuming that you have some raw materials which have been in the store for some periods of time.  A few months later, some were found to be faulty.  So you need to return back the materials to your vendor but it does not relate to any particular PO.

Vendor Returns Without PO Reference

You can used Return Purchase Order, transaction code ME21N

At the item details, look for the Return columns and tick it.

MIGO_GR - Goods Receipt for Return Purchase Order
Movement type will be 161 to deduct the stock and 162 for reversal.
During Goods Receipt for Return Purchase Order, you do not have to change the movement type from 101 to 161 as the system will automatically assign the movement type to 161 upon saving the postings.  However, before saving, check if there is a tick in the Return Column to ensure that it is a return Purchase Order.

Create a new Purchase Order Number range to differentiate
OMH6 - Define Number Ranges

Create a new Purchase Order Type to differentiate e.g ZB
OMEC - Define Document Type

Changing the PO Layout sets
OMFE - Messages: Output Programs

/:  IF &EKKO-BSART& = 'ZB'.
/       RETURNED PURCHASE ORDER
:   ELSE.
/       PURCHASE ORDER
/:  ENDIF.

SAP MM Reference Books:
SAP Material Management Interview Questions, Certification and Configuration Books

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