(6) The books of account or other documents seized under sub- section (1) or sub- section (2) shall not be retained by the authorised officer for a period exceeding one hundred and eighty days from the date of the seizure unless the reasons for retaining the same are recorded by him in writing and the approval of the Commissioner for such retention is obtained: Provided that the Commissioner shall not authorise the retention of the books of account or other documents for a period exceeding thirty days after all the proceedings under this Act in respect of the years for which the books of account or other documents are relevant are completed.