(b) where the duty is not so adjusted either for the reason that the goods are not exported within the period specified in the said Advance Licence, or within such extended period not exceeding six months as the 1[ Assistant Commissionr of Customs] may on sufficient cause being shown, allow, or for, any other sufficient reason, the importer shall, notwithsatn- ding anything contained in section 28, be liable to pay the amount of duty not so adjusted together with simple interest thereon at the rate of twelve per cent. per annum from the date the said permission for clearance is given to the date of payment.