Advance received

SIVAPRAKASH K (20 Points)

13 August 2019  
we received advance from a customer in Aug 2017 and declared in Gstr1 but not in Gstr3b. The customer was billed in sept 2017 and paid the GST. but did not reverse the advance. we found this error while reconciling data for filing annual return.Is it possible to reverse the advance now or how to adjust it ?
Thanks