There will be times when items are received and they should not be, most often it will be from misuse of the "receive all" icon or just an honest mistake. Trx does have a screen that will reverse some of those mistakes. If a purchase order was an inventory order, that means it has no project items or drop ship, gen PO or Auto PO items line items from that PO can be reversed.
PUR > PO Receiving Processing > Reverse PO Receipt 
The key to this screen is the Purchase Order must not be tied to an order or a project. As long as it is an inventory purchase order individual items can be reversed as needed. This reversal will reverse the GL entries for those items as well as the inventory amounts.
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