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.
- Purchase order – The purchase order number that has item that you would like to reverse on it the vendor and PO date will automatically populate in.
- Rev Waybill – A Purchase order can have many waybills if it was received a different time the first one will populate in to the screen but you can type another or press F9 to choose from a list of available waybills.
- Date – This is the date that the items will be un-received, the current date will default in.
- Quantity Received – This is the only part of the line item detail that you can adjust, place a negative amount for the items that you would like to un-receive. If you would like no change add a zero.
- Clear – the clear icon will remove the detail from the line items and the screen.
What to do if Purchase order is tied to an order.
Order – If the item is tied to a Drop Ship, Gen or Auto PO you will not be able to use this screen and you will be forces to do this the un-automated way via vendor RMA. Essentially, when the item was received you told TRX that the item is now in your inventory. To reverse it out you will need to do a Vendor RMA and sent it back to the vendor an in some cases create a new PO for that item.