Answer:
No, FusionRetail6 does not support discount calculation in such a way in Prepaid Mode. In Prepaid Mode, Customer card discount is calculated after the selection of the product.
So, the card must have the balance; though you can pass the same transaction in Sale Invoice (Scan POS).
For more details please visit: http://www.rancelab.com/help/fr6_prepaid_card_transaction.htm