Provisional assessment of customs duty permits temporary duty fixing, later adjustment, and interest on delayed payment or refund. Provision is made for provisional assessment of customs duty where self-assessment cannot be completed, goods require testing, further enquiry is needed, or documents and information remain pending. The proper officer may assess duty provisionally on security for any deficiency, must finalise the assessment within the prescribed period, and may extend that period in limited cases. On final assessment or re-assessment, provisional payments are adjusted, deficiencies are payable, excess amounts are refundable, and interest applies on delayed payments and refunds.