Yes you may return the item to supplier after delivery, as the received item may not be of good quality and doesn't meet your expectations.
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Yes, you can do the RTV even after delivery the items into Inventory.
I believe this functionality provided to support the returning of goods to vendor in case material is damaged or not fulfilling our expectation.
Hope it is clear now.
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Yes you may return the material back to the supplier even after delivered. Use Returns form and query with the PO number and enter return for the Inventory Item. Use Misc Receipt to return from expense location.