Cash discount amount cannot be posted. See long text

What causes this issue?

The following situations could have led to this error:
Situation 1:
You want to settle a purchase order that contains only returns items.
Since these items are posted as credits, you must settle them as a
credit memo using the transaction MIRO. However, you have used the
transaction MIRO and posted an invoice document.
Situation 2:
You entered either an invoice or a credit memo containing items to be
debited (=invoice items) and items to be credited (=credit memo items).
If you are entering a credit memo, the following situation has
occurred:
The amount eligible for cash discount in the relevant invoice items (
&V1& &V3&) is larger than the amount in the relevant credit memo items
(&V2& &V3&). This means that a negative amount is posted as the cash
discount amount.
If you are entering an invoice, the following situation has occurred:
The amount eligible for cash discount in the relevant credit memo items
(&V2& &V3&) is larger than the amount in the relevant invoice items (
&V1& &V3&) This means that a positive amount is posted as the cash
discount.

System Response

Situation 1:
The system is not able to settle the purchase order.
Situation 2:
If you are entering a credit memo:
The system cannot post a negative cash discount amount.
If you are entering an invoice:
The system cannot post a positive cash discount amount.

How to fix this error?

Situation 1:
Exit the current transaction.
Enter and post the purchase order using the credit memo transaction in
transaction MIRO.
Note that you must settle the returns items as credit memo items.
Select this option in <LS>More allocation criteria</>.
Situation 2:
Exit the current transaction. (If you have already saved the invoice or
credit memo, delete it.)
Enter and post the credit memo and the invoice separately; note the
following when doing this:
Only assign items to be credited to the credit memo.
Only assign items to be debited to the invoice.

Error message extract from SAP system. Copyright SAP SE.