Issuing b2b invoice to the customer,for which we have already issued b2cs invoice

Dear All,

Scenario : 

Supplier  in Kerala Buyer  in Orissa

Issuing B2B invoice to the customer,for which we have already issued B2CS invoice

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Dealer in Kerla sold tread rubber to Orissa in  the months of June,July 2018. The outward supplies for the two months was approximately Rs.10 Lakh.

On September  2018 buyer in Orissa told they have registration and they want GST Invoice for their purchase. 

How ever the supplier in Kerala has already sold the materials considering them as Unregistered Party and the GSTR-1 till August 2018 have been filed. 

It all happened due to the  mistake  by the agent who was the link between the two parities who forgot to provide us the GSTIN of the buyer and Is there any possible solution by which the seller in Kerala can rectify the error ?

Kindly suggest your views.