Partial credit notes cannot be created via PlentyONE and reported to OTTO.
The sequence for importing your partial credit notes is as follows:

1. You manually create the partial credit note in the OPC as desired by you
   → OTTO creates the document for this, so you also need this in PlentyONE
2. Now you create an order of type credit note in PlentyONE and adjust the prices according to the partial credit note at OTTO
   → Important: the event action for accepting the return must NOT be triggered!
3. Then you provide us with the credit note ID in a ticket for our technical support and we will import the document.