Mistakenly shown credit amount in GSTR 3B return

I was mistakenly took credit of IGST in GSTR 3B whiling filing of return in the month of July, 17 and waited for an option of amendment of GSTR 3B to reverse the same, as I was information that govt. would introduce amendment in GSTR 3B. Further on December, 17 3B return, I reversed all the credit amount of IGST. Now the Tax Dept. is demanding interest @ 24% for the period from taking excess credit to reversing the said credit. It was my just my typing mistake. Now what should I do?