Refer below code.

protected void OnSave(object sender, EventArgs e)
{
    if (Page.IsValid)
    {

        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[] { new DataColumn("Item"), new DataColumn("Price"), new DataColumn("Quantity") });
        foreach (Control c in ph1.Controls)
        {
            if (c.GetType().Name.ToLower() == "usercontrol_ascx")
            {
                UserControl uc = (UserControl)c;

                TextBox tbItem = uc.FindControl("txtItem") as TextBox;
                TextBox tbPrice = uc.FindControl("txtPrice") as TextBox;
                TextBox tqty = uc.FindControl("txtQuantity") as TextBox;
                if (!string.IsNullOrEmpty(tbItem.Text.Trim()) && !string.IsNullOrEmpty(tbPrice.Text.Trim()) && !string.IsNullOrEmpty(tqty.Text.Trim()))
                {
                    dt.Rows.Add(tbItem.Text.Trim(), tbPrice.Text.Trim(), tqty.Text.Trim());
                    if (Convert.ToInt32(tqty.Text.Trim()) > 0)
                    {
                        int availableQuantity = Convert.ToInt32(GetAvailableQuantity(tbItem.Text.Trim()));
                        if (Convert.ToInt32(tqty.Text.Trim()) > availableQuantity)
                        {
                            ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('No of Quantity entered is not available in Stock')", true);
                        }
                        else
                        {
                            int inserted = Insert(tbItem.Text.Trim(), tbPrice.Text.Trim(), tqty.Text.Trim());
                            if (inserted > 0)
                            {
                                UpdateStock(tbItem.Text.Trim(), tqty.Text.Trim());
                                ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Data Submitted Successfully ... !!')", true);
                            }
                        }
                    }
                }
            }
        }
        // BindGridID();
        gvInsertedRecords.DataSource = dt;
        gvInsertedRecords.DataBind();
    }
}

private string GetAvailableQuantity(string item)
{
    string availableQuantity = "";
    string constr = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        string query = "SELECT Qty FROM Stock WHERE Item = @Item";
        using (SqlCommand cmd = new SqlCommand(query))
        {
            cmd.Connection = con;
            cmd.Parameters.AddWithValue("@Item", item);
            con.Open();
            availableQuantity = Convert.ToString(cmd.ExecuteScalar());
            con.Close();
        }
    }

    return availableQuantity;
}

private int Insert(string item, string price, string qty)
{
    int i = 0;
    string constr = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        string query = "INSERT INTO ItemOrdered (Store,SellsPerson,Receipt,Item,Price,Quantity,Sum) VALUES (@Store,@SellsPerson,@Receipt,@Item,@Price,@Quantity,@Sum)";
        using (SqlCommand cmd = new SqlCommand(query))
        {
            cmd.Connection = con;
            cmd.Parameters.AddWithValue("@Store", Department.SelectedItem.Text);
            cmd.Parameters.AddWithValue("@SellsPerson", HttpContext.Current.User.Identity.Name);
            cmd.Parameters.AddWithValue("@Receipt", txtrecipt.Text);
            cmd.Parameters.AddWithValue("@Item", item);
            cmd.Parameters.AddWithValue("@Price", price);
            cmd.Parameters.AddWithValue("@Quantity", qty);
            cmd.Parameters.AddWithValue("@Sum", Convert.ToDecimal(price) * Convert.ToDecimal(qty));
            con.Open();
            i = cmd.ExecuteNonQuery();
            con.Close();
        }

        return i;

    }
}

private void UpdateStock(string item, string qty)
{
    string constr = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        string query = "UPDATE Stock SET Qty = Qty - @Quantity WHERE Item = @Item";
        using (SqlCommand cmd = new SqlCommand(query))
        {
            cmd.Connection = con;
            cmd.Parameters.AddWithValue("@Item", item);
            cmd.Parameters.AddWithValue("@Quantity", qty);
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }
}