Save as otherwise provided in sections 30 and 41, the proper officer may, in his discretion, authorise any document, after it has been presented in the custom house to be amended ¹[in such form and manner, within such time, subject to such restrictions and conditions, as may be prescribed]:
     Provided that no amendment of a bill of entry or a shipping bill or bill of export shall be so authorised to be amended after the imported goods have been cleared for home consumption or deposited in a warehouse, or the export goods have been exported, except on the basis of documentary evidence which was in existence at the time the goods were cleared, deposited or exported, as the case may be.