Dear Borgerink,
Here the balance 1500 is not paid and is still a debit balance. Debit balance can be picked up by payment run only if you choose a payment method with incoming payment set. Only credit balances are paid via payment method with outgoing payment method. So I see no issue in business process as well. Hope this explanation is more clear else please feel free to reach me back.
Also for your reference, create a F110 run only with a payment method which is set as outgoing payment and recreate the same case above you can see that it will fail saying debit balance.
Note that for both invoice and credit memo to be clubbed together all the fields as mentioned earlier should be same.
Kind Regards
Soumya Rao
