Debit note /credit note

Allauddin patel (Accountant ) (111 Points)

22 June 2020  
dear sir...
I had sold good to one of my party in the month of April 2020 and when party received goods he found some damage goods in that and he (recipient) issued debit note to us by reducing the invoice value in same month April. and he sent that debit note to us by mail on 20 June.....
so pls tell me what should we do now.... ???

I have filled GSTR 1 and GSTR 3b of April month according to my original invoice....

please solve this my problem