Hi again Stefan,
After connecting with our product team on this, it looks like the issue you’re experiencing is because you’re creating a return for an item that’s already part of another return (in the requested state). In the second example (through the native admin) you’re able to complete the flow successfully because the item to be returned is not in another return. You cannot add exchange items to a requested return via API today - instead you should use the returnCreate mutation.
If this mutation/ flow doesn’t work for you, can you explain why you want to add exchange items to a requested return, rather than creating a return given you have a return app that seems to approve/decline the returns?