Since Ivy is a project-based system which allows you to create POs for the same vendor across many projects, you are unable to create credits from PO payments in Ivy.
If a PO is already fully paid, and you need to reflect a vendor credit or a refund/return from a vendor on the PO, you will first need to edit the manually recorded payment to reflect the reduced amount you ended up paying to your vendor. You can do this by viewing the payment details on the PO, and clicking on the pencil button located next to the payment record. You'll be able to update the payment amount at that point. For online payments made via Ivy on the PO, please contact firstname.lastname@example.org for assistance with editing the online payment record.
Once you have reduced the payment amount on the PO, you will be able to edit total/balance of the PO by editing the relevant line items, and reducing the cost of the item as needed. Once you reduce the cost of the item on the PO, you'll see that the total balance of the PO will be reduced as well.
We recommend including a note on the memo or on the edited line item to reflect why and when the change in cost occurred. You would then need to manage your credit outside of Ivy, directly with the vendor.