Sale bill in gst

Dear Sir,
I submitted sale and purchase details to our gst consultant for submitting gst. in the month of august i sens the same as usual, but there was a incidents happened that my clint returned a sale bill for a small mistake on 13rh august and i found that there was a mistake and net amount will be decrease by 300 rupees. i talk to consultant he said that sale bill already submitted on 7th august you have to adjust the amount in next month. pl suggest how do i adjust the amount in next month.