Penalty Payable under Section 129(1a) - Goods and Services Tax - GST

Goods Intercepted during Transit On which Applicable Tax Rate is 18% while Tax Invoice Raised for 12% Tax Rate. Therefore Invoice Raised for Tax Value of Rs. 100 on which Tax Paid is 12. E-way Has been made Correctly. Now Calculation of 200% penalty for Tax payable will be Rs. 36 or Rs. 12 Case 1 : Tax Payable on 100 will Rs. 18 while already paid Tax is 12. Therefore Tax Short Paid is 6 on which 200% penalty will be 12 Case 2: Tax payable on Rs. 100 is 18 therefore Penalty under 129(1a) is 36 What does the GST Law/Rules says on this?