B2cs invoice amendment

we have wrongly return filed for September-2017 with 18% tax insteated of 12%, now we are doing amendment with 12% in August-2018 in Gstr-1 returns again liability showing in b2cs amendment section. kindly advise, example September-17 B2cs sales filed 1000 @ 18%= 180, now we are doing amendment in august-18 1000 @ 12%=120, i have already paid 60 rupees excess tax, but B2cs section showing again 120 rupees liability.