Sales in ITR

Dear Sir,

One of my client took gst no. in Oct 2020. His Turnover in GST Return is 9 lakhs. While, in his bank, Credit is 20 lakhs because of Sale (01.04.20 to 30.09.20)

If I show 20 lakh Sale in ITR, it would be mismatch from GST Returns. and may attract notice from GST. What to do and how to SHOW in ITR to avoid such mismatch notice??

or Should i drop ITR filing this year. Because, its his first ITR??