Hi Eri,
You have a couple options:
1.) If the PO has been received but not invoiced, and if the Items are still in the Receiving bin, then you can still undo the receipt from the Posted Purchase Receipt. To do that, you open the Posted Purchase Receipt, highlight the line to work with, and click on Functions => Undo Receipt. You may need to delete the Put-Away first if you use "Directed Pick and Put-Away" (this is a warehouse add-on).
After you undo the receipt, you can go to the PO and lower the Quantity ordered to however many were previously received and paid for (if applicable) so the PO has nothing left outstanding, or you can simply set the Quantity to zero, then archive, and delete the PO.
If the PO has been received but not invoiced, and if the items have already been consumed and no longer reside in the Receiving bin, then it may be more difficult. You can try to on-hand adjust the product back into the Receiving bin for removal using the Undo Receipt function. If that doesn't work try the next option.
2.) If the PO has been received and you can't reverse the receipt, then you have to Invoice the receipt and do a Purchase Credit Memo to reverse the payment (same if it has already been invoiced). When you do a Purchase Credit Memo there are different scenarios too:
A.) If you want to reverse the payment and also reduce the inventory by the amount received then you can use the Purchase Return Order process.
B.) If you want to reverse the payment but not affect the inventory then you can use a Purchase Credit Memo without doing a Purchase Return Order. When you do a Purchase Credit Memo, there are 2 different scenarios to that:
I.) You can fill out the Purchase Credit Memo Lines section using the "G/L Account" line Type to do a payment reversal without affecting inventory.
II.) You can fill out the Purchase Credit Memo Lines section using the "Item" line Type. When you post the Purchase Credit Memo NAV will deduct the same Item quantities from the On-Hand Inventory because you used actual Item numbers.
Note: If you use "Directed Pick and Put-Away" or notice that your Bin Content quantities don't match your On-Hand quantity then you may have to use the Item Journal and the "Calculate Whse Adjustment" for each of the items that were on the Credit Memo to re-synchronize the On-Hand inventory amount with the Bin Content amount total.