Hi Viraj,
The discount functionality that you are looking for is not possible through Standard SAP, as Standard FICA payment terms only support one level of cash discount. You have to cater your requirement through an enhancement.
If you are willing to post the discount as a separate document, then you have to enhance event 0020.
Else, if you are willing to post the discount as an additional item against the payment document, then you have to enhance event 0032.
Hope it helps.
Thanks,
Amlan