Credit note more than sales value in oct-18

Dear Raja Sir,

We issued an export service invoice in the month of July -18 and shown in GSTR- 1 as well as GSTR-3B, Due to some problem of our work the client has rejected our invoice , so we  passed the credit note for reversal of the invoice in Oct-18 . Now the problem is in Oct-18 we have a small service bill and the credit note figure is more than the sales bill. 

Pls advice how to show in GSTR-3B ( as negative figure not accepted in GSTR-3B).

Pls note we have taken LUT so there is no question in GST liability.

Thanks in advance 

Waiting your valuable advice

Regards

Gupta