Doubts on discount after billing.

it can be apply within 6 months please check 6 months expired or not.

it should be claimed by revised the vat return

yes, by revised return. you can also used credit note for that purpose with separately  write basic amount & tax

You can issue credit note to the purchaser for discount amount and you have to revised your monthly return respectively within 6 month for which monthly return you want to revised. for the example:-

if u want to revised your return of January' 2012 you have to revised it before 20th of August' 2012