There are many, many reasons why you return product to a vendor. Defects, mistake or error on receiving, etc. etc.
Currently, the return uses the MAC for the return value. Doesn't make sense. The vendor will either refund what I paid them or an agreed upon price. I can set the price on the return, but the moving average cost (MAC) isn't updated. If it's not updated, there should be a variance posted to the GL for the difference, but currently no variance is created.
What really should happen, since the return is item specific, is the value of the inventory (MAC) should be recalculated by the return. The system used the original receipt to obtain the value/MAC, it should also play out in reverse. Think about a user mistakenly receiving 1,000,000 pieces versus 100,000. Best way to fix is reverse the receipt and re-receive. Acumatica leaves the MAC derived from the mistake.