B2B sales in Aug 2019 but not filed GST 1 and 3B

Hi
one of our client have done b2b sales in Aug 2019 but have not filed the GST returns . The customer claimed the ITC

we have filed the nil returns from Aug 19 to Dec 19 in Jan 20, though there is sales in Aug 19.
Now we want to rectify the error. do we get any penalty. please advise