Your code didn't filter against your datagridview datasource.
This line just creates an empty DataTable and then you filter:
DataTable dt1 = new DataTable();
DataView dv = dt1.DefaultView;
dv.RowFilter = string.Format("Series LIKE '%{0}%'", cmbBillSeries.SelectedItem.ToString());
You can try this to filter against datagridview datasource:
(grdSpanSale.DataSource as DataTable).DefaultView.RowFilter = string.Format("Series LIKE '%{0}%'", cmbBillSeries.SelectedItem.ToString());