GST on freight charged in invoice

I am selling goods from Madurai to Chennai. I am raising sale invoice for  cost of materials as Rs. 1000 and freight Rs. 100 and paying GST on Rs.1100. The buyer has paid Rs 100 paid the freight to lorry driver by debiting my account as advance and he is paying the cost + GST. Now i have to credit the freight amount to the buyer account by debiting freight account. In this situation kindly confirm whether I have to pay GST under RCM for the freight amount or the GST payment on freight has paid thro earlier sales invoice is enough.  kindly confirm.