'// ..
  Try
        Da.Fill(Dt)
        Catch ex As Exception
            Debug.WriteLine(ex.ToString())
        End Try
        Dim sum As Integer = CInt(dt.Compute("SUM(Einnahme)", String.Empty)) 
        txtEinnahmen.Text = Format(sum, "#,##0.00")
'// ..

'// ..
Try
        Da.Fill(Dt)
        Catch ex As Exception
            Debug.WriteLine(ex.ToString())
        End Try
        Dim sSql As String = "Select SUM(Einnahme) As Summe From Kassenbuch"
        Dim cmd As New OleDb.OleDbCommand(sSql, cn)
        Cmd.Connection.Open()
        Dim sum As Integer = CInt(cmd.ExecuteScalar())
        txtEinnahmen.Text = Format(sum, "#,##0.00")
        Cmd.Connection.Close()
'// ..