Custom duty - Goods and Services Tax - GST

Dear Professional brothers Sisters My client has import the goods during the financial year 2018-19 from out of Indian Territory, after verification of goods by the custom authority as assessed the duty my client has paid (IGST) @12% and custom authority released the goods. Now after more the six year custom authority issued a notice to my client on the grounds that you have liable to pay duty @ 18%. After inspection of the file of the department there is no any sample of goods attested in the file. Can after such a long time can department asked to pay the difference of duty when the sample of goods also not on file. With regards J.S. Uppal