Conversion Of Int Data Type Error?!

 Hi,
I keep getting the error:
System.Data.SqlClient.SqlException: Conversion failed when converting the varchar value '@qty' to data type int.
 When I initiate the insert and update.

I tried adding a: Convert.ToInt32(TextBox1.Text), but it didn't work.. 
Could someone help?
My code:
private bool ExecuteUpdate(int quantity)
{
  SqlConnection con = new SqlConnection();
  con.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";


  con.Open();

  SqlCommand command = new SqlCommand();
  command.Connection = con;
  TextBox TextBox1 = (TextBox)FormView1.FindControl("TextBox1");
  Label labname = (Label)FormView1.FindControl("Label3");
  Label labid = (Label)FormView1.FindControl("Label13");

  command.CommandText = "UPDATE Items SET Quantityavailable = Quantityavailable - '@qty' WHERE productID=@productID";
  command.Parameters.Add("@qty", TextBox1.Text);
  command.Parameters.Add("@productID", labid.Text);
 command.ExecuteNonQuery();

  con.Close();
  return true;
}


    private bool ExecuteInsert(String quantity)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";

        con.Open();

        SqlCommand command = new SqlCommand();
        command.Connection = con;
        TextBox TextBox1 = (TextBox)FormView1.FindControl("TextBox1");
        Label labname = (Label)FormView1.FindControl("Label3");
        Label labid = (Label)FormView1.FindControl("Label13");

        command.CommandText = "INSERT INTO Transactions (Usersname,Itemid,itemname,Date,Qty) VALUES (@User,@productID,@Itemsname,@date,@qty)";
        command.Parameters.Add("@User", System.Web.HttpContext.Current.User.Identity.Name);
        command.Parameters.Add("@Itemsname", labname.Text);
        command.Parameters.Add("@productID", labid.Text);
        command.Parameters.Add("@qty", Convert.ToInt32(TextBox1.Text));
        command.Parameters.Add("@date", DateTime.Now.ToString());
        command.ExecuteNonQuery();

        con.Close();
        return true;
    }


protected void Button2_Click(object sender, EventArgs e)
{
  TextBox TextBox1 = FormView1.FindControl("TextBox1") as TextBox;
  ExecuteUpdate(Int32.Parse(TextBox1.Text) );
}

protected void Button2_Command(object sender, CommandEventArgs e)
    {
        if (e.CommandName == "Update")
        {
            TextBox TextBox1 = FormView1.FindControl("TextBox1") as TextBox;
            ExecuteInsert(TextBox1.Text);
        }
    }
 
 
Thanks so much if someone can!
Jon