Provisional duty assessment permits provisional taxation when a bond and directed deposit secure potential final duty. If the proper officer cannot make a final assessment, he shall estimate the duty most likely to be levied as the provisional duty. The importer or exporter may secure a provisional assessment by executing a bond equal to the difference between the potential final duty and the provisional duty and by depositing with the proper officer a directed sum not exceeding a proportion of the provisional duty; thereafter the proper officer may provisionally assess the goods at the provisional duty.