Procedure for replacement of defective goods sold

Dear Experts,

Please furnish the procedure to be followed to replace the defective (quality issue) goods billed and consigned to the customer under tnvat and excise rules.

Also please clarify

1.whether the replacement can be made before the defective goods are collected and

2. is there any issue if the defective goods are collected subsequent to the replacement

Regards,

Govindarajan