RCM on transportation due to wrong reporting by GTA - Goods and Services Tax - GST

Dear experts, My client sold goods to unregistered recepient.Freight paid by the recepient.But the transporter(GTA) while filing GSTR-1 ticked GST payable on RCM mentioned GST no.of our client.Now while doing GST audit the department is asking for RCM on this transaction.Plz guide how we are liable for such RCM how to pleade for that.