Credit Note error after sales return transaction

Hello Sirs; Good day to you. I noticed whenever I make a transaction and attempt to follow up with a credit note; the system will not submit; it pops an error that the outstanding for SINV -xxx can not be less than 0 (-amount). Does this mean that credit notes are not allowable for return sales against paid invoices? And why, is there something that needs to be done. This is a critical sales function; please guide.

As per the current design it seems you cannot book a credit note against a paid invoice. The invoice reference is for the invoice (future?) that you will cancel the credit note against.

You can add the invoice reference as a custom field, or as a note / comment.