For Spanish localization, the only option to correct Free text invoices is by using 'Credit invoicing' in the Free text invoice window.

To use the 'Credit invoicing' is necessary to first create a new Free text invoice, than add a new line with negative quantity (negative amount too) and only after this the option will be enable. If a negative quantity and amount is not appearing the 'Credit invoicing' button is disable.

If we click on the button 'Credit invoicing', we can associate an invoice (any invoice) and then we need to choose 'Reason code', 'Reason for correction', 'Correction method'. After this we can post this Free text invoice (with negative quantity):

Ideas:
1. Update documentation regarding Free text invoices. It is only related to purchase invoice/credit note - Create a corrective invoice (Spain) - Finance | Dynamics 365 | Microsoft Docs

2. On the free text invoice list or on the Customer transactions there isn't any reference to the invoice associated with the credit note. There isn't also a note saying it was corrected. It is not possible to immediately have this information.
The idea is to create a simply/faster way to have this information, otherwise with so many invoices it is really difficult to know which ones are associated with credit notes.
Please note field ‘Correction status’ is not populated - no immediate reference exists to inform the user of the document correction
The only option is to click on the button 'View the customer transactions related to credit invoicing', on Customer transactions form.