Error: 'This SqlTransaction has completed it is no longer usable' in ASP.Net

I found the solution. The problem was the position of the conditional statement.

What i did was to move the conditional statement up like this:

Public Sub SaveCustomerData()
    Dim constr As String = ConfigurationManager.ConnectionStrings("ConnString").ConnectionString
    Dim transaction As SqlTransaction
    Using con As New SqlConnection(constr)
        con.Open()
        transaction = con.BeginTransaction()
            Try
                Dim id As Integer = 0
                If Label1.Text <> String.Empty AndAlso Label1.Text IsNot Nothing Then
            Using cmd As New SqlCommand("PR", con, transaction)
                cmd.Parameters.AddWithValue("@GRNumber", "1233")
                cmd.Parameters.Add("@GRID", SqlDbType.BigInt).Direction = ParameterDirection.Output
                cmd.CommandType = CommandType.StoredProcedure
                cmd.ExecuteNonQuery()
                id = Convert.ToInt32(cmd.Parameters("@GRID").Value)
            End Using
            For Each row As GridViewRow In GView.Rows
                Dim ponum As String = DirectCast(row.Cells(1).FindControl("lblPONumber"), Label).Text
                Dim partyname As String = DirectCast(row.Cells(2).FindControl("lblPartyName"), Label).Text
                Dim itemname As String = DirectCast(row.Cells(3).FindControl("lblItemName"), Label).Text
                Dim qty As String = DirectCast(row.Cells(4).FindControl("lblQtyOrdered"), Label).Text
                Dim qtydelivered As String = DirectCast(row.Cells(5).FindControl("lblQtyDelivered"), Label).Text
   
        
                Using cmd As New SqlCommand("PR_Det", con, transaction)
                    cmd.Parameters.AddWithValue("@GRID", id)
                    cmd.Parameters.Add("@PONumber", SqlDbType.VarChar).Value = ponum
                    cmd.Parameters.Add("@PartyName", SqlDbType.VarChar).Value = partyname
                    cmd.Parameters.Add("@ItemName", SqlDbType.VarChar).Value = itemname
                    cmd.Parameters.AddWithValue("@QTYOrdered", qty).ToString()
                    cmd.Parameters.AddWithValue("@QTYReceived", qtydelivered).ToString()
                    cmd.CommandType = CommandType.StoredProcedure
                    cmd.ExecuteNonQuery()
                End Using
           Next
                    transaction.Commit()
                    
                End If
            Catch sqlError As SqlException
                transaction.Rollback()
            Finally
                con.Close()
            End Try
    End Using
End Sub

Note, the value of Label1.Text is gotten from edit/update button. once i click edit button the value of gridview column is passed to Label1.Text. You can see how the edit button works in the following thread: 

http://www.aspforums.net/Threads/787552/Fire-Dropdownlist-In-Gridview-Row-When-Edit-Button-Is-Clicked/?p=2

Thank you Shashikant and Dharmendr for your help.