Forgot to Report Invoice in September 2020

Hi there!

I was reconciling my GST returns again for 2019-20 and found that I had missed certain invoices that had to be rectified. Because of this I have not passed on credit of Rs. 15000 but have paid the same to the department.

Now as the return for September 2020 has been filed, what is the best way by which I can amend my invoice/provide credit to the party.

I tried using the Amendment option in the GST Return for December 2020 and there I was able to amend the invoice. Would this help in passing on the credit?