Retrospective amendment allows payment with certified input credit and deems past excise actions valid, subject to verification. Insertion of rule 57CCC retrospectively amends the Central Excise Rules, 1944. A person may opt to pay amounts with interest and apply to the Commissioner within six months, submitting documentary evidence and a Chartered Accountant or Cost Accountant certificate certifying input credit attributable to inputs used in exempt or nil-rated final products. The Commissioner must verify payment within two months and require any differential amount with interest to be paid within ten days. Actions taken from 1 September 1996 to 31 March 2000 regarding the amended provisions are deemed valid, the Central Government is vested with retrospective rule-making power for the subsection, and no new punishability is created by the section.