Retrospective amendment to CENVAT credit rules permits certified payment of input credit for exempted goods subject to verification and interest. Rule 6 of the CENVAT Credit Rules, 2001 is retrospectively amended per the Sixth Schedule. A person may opt to pay the amount due under the amendment with interest and must apply to the Commissioner within six months of presidential assent, providing documentary evidence and a Chartered Accountant or Cost Accountant certificate quantifying input credit attributable to exempted goods. The Commissioner shall verify payment within two months and require any differential plus interest to be paid within ten days. Actions during the specified retrospective period are declared valid, and the Central Government is deemed to have rulemaking power to enact retrospective rules for this purpose.