Forms Personalization when Discard Button is pressed for a Paid Invoice

Hi,

  I need to personalize the Invoice Screen in such a way when the Discard Line is Pressed against an each line for a paid Invoice, an error message needs to be displayed.

I have personalized that using the trigger event as WHEN-NEW-ITEM-INSTANCE and the Trigger Object as LINE_SUM_CONTROL.DISCARD_LINE and the condition have checked as 'Payment Amount' is Not Null. But, still am able to discard the lines of Paid Invoices.

Any help is highly appreciated.

Thanks,

Swathy