Repro Steps:

1. Create a new sales order and select a customer. 
2. Enter a payment method and specify a prepayment percentage. 
3. Add an item and set the quantity.
4. Post the prepayment invoice.
5. Attempt to change the invoice discount directly in the sales order; observe the error message. 
6. Try to change the invoice discount via the statistics; observe the error message. 
7. Post the customer payment using the cash receipt journal, applying it to the prepayment invoice. 
8. Open the sales order again, release it, and check the status. 
9. Attempt to change the invoice discount via the sales order card (should not be possible). 
10. Try to input an invoice discount amount via statistics and attempt to post the invoice. 

Actual Result:

• Error messages are shown when trying to change the invoice discount after posting the prepayment invoice, both directly and via statistics. 
• After payment and releasing the order, changing the invoice discount via the sales order card is not possible, but it is still possible via statistics, which should not be allowed. 
• Attempting to post the invoice after changing the discount via statistics results in an error message. 

Expected Result:

• It should not be possible to change the invoice discount via statistics after the prepayment invoice is posted and the order is released. 
• The system should consistently prevent changes to the invoice discount in all relevant areas once the prepayment process is completed.