Thanks for explaining what you're trying to accomplish, @Steph-WDSite.
Allow me to step into this conversation and provide clarifications about correcting a purchase order in QuickBooks Online.
Being able to link a credit to a purchase order isn't possible in QuickBooks Online. This process is only applicable to a bill. As mentioned by my peer @Charies_M above you'll need to create a new purchase order to track those items that needs to be returned. Afterwards, you can enter a memo on your purchase order to indicate the reason for the transaction.
For the steps in creating PO, please refer to the steps given above. Also here are some helpful articles about recording a vendor refund for your future reference:
Record a vendor refund in QuickBooks Desktop.
Accounts Payable workflows in QuickBooks Desktop.
That should do it.
Please let me know if you have any other questions about PO in QuickBooks Desktop. I'll be around to provide further assistance. Have a great day!
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