Credit note in gstr 1 without having taxable sale.

I have filed my gstr 1 for the the month of august with taxable sale only.
Now, I have to file September month return which is exempt sale only and having a credit note with regard to the sale made in August return.
I have submitted the Gstr 1 with exempt sale and taxable credit note but the issue is how will i get benefit of credit note.
If i have filed the credit note with taxable sale, outward tax would already been auto -populated net of credit note.
Now the case is different, so how would i take its effect in GSTR3B.

Thanks in advance for help.