i can't get 2 query in C# & SQLSERVER2008

      internal bool saveNewPayment(Guid ID, Guid creditID, decimal paymentAmount, DateTime dateTime)
      {
          bool flag = true;
          using (SqlConnection con = new SqlConnection(constring))
          {
              con.Open();
              SqlTransaction sqlTranc = con.BeginTransaction();
              SqlCommand com = con.CreateCommand();
              com.Transaction = sqlTranc;

              try
              {
                  string payAmount = paymentAmount.ToString(CultureInfo.InvariantCulture.NumberFormat);


                 string query = string.Format("INSERT INTO Payment (ID, CreditsID, Amount, PaymenDate)" + " VALUES ('{0}', '{1}', '{2}', '{3}')",
                   ID, creditID, payAmount, dateTime.ToString("MM/dd/yyyy"));

                  com.CommandText = query;
                  com.ExecuteNonQuery();

                 query = string.Format("UPDATE Credits SET Balance = (Balance - {0}) WHERE ID = '{1}'", payAmount, creditID);

                  com.CommandText = query;
                  com.ExecuteNonQuery();


                  sqlTranc.Commit();

              }
              catch (Exception)
              {
                  sqlTranc.Rollback();
                  flag = false;
              }
              finally
              {
                  if (con.State == System.Data.ConnectionState.Open)
                      con.Dispose();
              }
          }
          return flag;

      }
  }
}