i entered the sales return the balance is never change .

if i entered the sales return the balance is never change please refer my code in (Button1_Click) and desing.

if i click the to return i need to add balance + return its cuming but in that label it show as 9

my design

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="purchasereturn.aspx.cs" Inherits="purchasereturn" %>

my coding

using System;

using System.Collections;

using System.Configuration;

using System.Data;

using System.Linq;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.HtmlControls;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Xml.Linq;

using System.Data.SqlClient;

public partial class purchasereturn : System.Web.UI.Page

{

SqlConnection con;

SqlDataAdapter sda;

protected void Page_Load(object sender, EventArgs e)

{

con=new SqlConnection("Data Source=SELVI-PC\\SQLEXPRESS;Initial Catalog=demo;Persist Security Info=True;Integrated Security=True");

//sda = new SqlDataAdapter("select balance from purchase where barcode=" + DropDownList1.SelectedItem.Text.ToString(), con);

}

protected void Button1_Click(object sender, EventArgs e)

{

Session["balance"]= TextBox4.Text;

try

{

if (TextBox5.Text != "")

{

TextBox4.Text = (Convert.ToInt32(TextBox4.Text) + Convert.ToInt32(TextBox5.Text)).ToString();

Label6.Text = "Our Balnce stock is" + (Session["balance"].ToString());

// DataSet ds = new DataSet();

//sda = new SqlDataAdapter("select balance from purchase where barcode=" + DropDownList1.SelectedItem.Text.ToString(), con);

//sda.Fill(ds);

//// Session["balance"] = TextBox4.Text;

// TextBox4.Text = ds.Tables[0].Rows[0]["balance"].ToString();

//Label6.Text = "Our Balnceis" + ds.Tables[0].Rows[0]["balance"].ToString();

//Label6.Text = "our bal is" + TextBox4.Text.ToString();

}

else if(TextBox5.Text=="")

{

Label6.Text = "Our Balnce stock is"+ (Session["balance"].ToString());

}

}

catch

{

}

}

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)

{

if (DropDownList1.SelectedIndex > 0)

{

DataSet ds = new DataSet();

try

{

con.Open();

sda = new SqlDataAdapter("select balance from purchase where barcode=" + DropDownList1.SelectedItem.Text.ToString(), con);

sda.Fill(ds);

TextBox4.Text = ds.Tables[0].Rows[0]["balance"].ToString();

}

catch

{

}

}

}

protected void TextBox5_TextChanged(object sender, EventArgs e)

{

if (TextBox5.Text!= "")

{

Label6.Text = "Our Balnce stock is" + (Session["balance"].ToString());

}

else

{

}

}

}