Reverse gst on non-payment of invoice

I had made Invoice in August for a client for a service and also paid GST while filing GSTR 3B of August.

But client company is shutting down now and even after 3 months client did not make payment and now refusing to pay.

So my question is

When filing GSTR1 for August - can I simply NOT add the bill? (but there will be break in serial number - is that allowed?)

I would prefer this option as then I do not have to do anything else.

OR

I must first show the bill in August GSTR1 and then show it as cancelled / debit note in October GSTR 1

With 2nd option - I do not know when will due date of filing GSTR 1 come which means I wont get GST back for atleast 3-4 months.

And then I also have risk of client rejecting debit note and adding complications to issue.

My main problem is that I will have recover about 8000Rs of tax which he must have claimed in his GSTR 3B of august.

So effectively I will not only get payment of services I gave but client will also siphon off my additional 8000Rs by claiming it as ITC.

What are dispute provisions in GST for such issues?