I found the solution, It will be helpful for someone
CancelItemsRequest cma = new CancelItemsRequest(System.Web.Configuration.WebConfigurationManager.AppSettings["GoogleMerchantID"].ToString(), System.Web.Configuration.WebConfigurationManager.AppSettings["GoogleMerchantKey"].ToString(), EnvironmentType.Sandbox.ToString(), ["merchant_invoice_id"], "Message", "");
cma.AddMerchantItemId("2"); // item id to cancel
cma.MerchantID = System.Web.Configuration.WebConfigurationManager.AppSettings["GoogleMerchantID"].ToString();
cma.MerchantKey = System.Web.Configuration.WebConfigurationManager.AppSettings["GoogleMerchantKey"].ToString();
cma.Environment = EnvironmentType.Sandbox;
cma.SendEmail = false;
GCheckoutResponse Respa = cma.Send();