GST difference

Hello expert One of my client return pre gst goods with gst and asking difference amount which he has paid to government. Is it right? If yes then how to enter our books of account.Example:We had sale 100 2(cst)=102 and he has return 100 12(igst)=112 and asking difference (12-2=10) amount which he has paid excess due to higher rate.Waiting for your response