Hi Abdalrahman Mohammed,
The discount amount fields can be personalized and shown in the customer payment journal.
I guess that is what you did by adding the ledgerjournaltrans.discount fields, right?
Those fields are however not editable in the customer payment journal and do not hold a value even if the invoice has a payment cash discount associated.
Note: If you enter vendor invoices with a cash discount the ledgerjournaltrans.discount fields are filled but that is a different functionality and relates to a different process & module.
Regarding the normal and always field selection:
If you specified a cash discount with the invoice and provided that the payment date is not after the cash discount date, a cash discount will automatically be shown in the cash discount fields in the settlement form.
Does this answer your question or is there still something open that needs clarification?
Best regards,
Ludwig