Check if entered quantity is greater than available stock in ASP.Net using C# and VB.Net

Hi

after modifying the update code i noticed that the code doesn't check if the qty inserted is greater than the stock and if so this line of code supposed to fire to the user but what i see is that the clientscript that shows when the inserted qty is greater than the stock fires it still submits the update with minus  like -2

ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('No of Quantity entered is not available in Stock')", true);

complete code

string model = ddlbrand.SelectedItem.Text.Trim();
 
// decimal unitPrice = Convert.ToDecimal(txtprice.Text.Trim());
int availableQuantity = Convert.ToInt32(txtstock.Text.Trim());
int quantity = Convert.ToInt32(txtqty.Text.Trim());
int inserted = 0;
 
//  int availableInventoryQty = AvailableInventoryQty();
if (txtqty.Text.Length > 0 && txtstock.Text.Length > 0)
{
 
    double stock = Convert.ToDouble(txtstock.Text);
    double customerqty = Convert.ToDouble(txtqty.Text);
    double tqty = stock - customerqty;
    double gqty = tqty;
    {
        {
            if (availableQuantity > 0 && quantity > 0)
            {
                using (SqlConnection con = new SqlConnection())
                {
                    con.ConnectionString = str;
                    using (SqlCommand cmd = new SqlCommand())
                    {
                        cmd.CommandType = CommandType.Text;
                        cmd.Connection = con;
 
 
                        cmd.CommandText = "INSERT INTO Damaged_Table (Store,Item, Stock,Qty_Damaged) VALUES(@Store,@Item, @Stock,@Qty_Damaged)";
                        // cmd.Parameters.AddWithValue("@SellsPerson", HttpContext.Current.User.Identity.Name);
                        cmd.Parameters.AddWithValue("@Store", Department.SelectedItem.Text.Trim());
                        cmd.Parameters.AddWithValue("@Item", ddlbrand.SelectedItem.Text.Trim());
                        // cmd.Parameters.AddWithValue("@Item", ddlitem.SelectedItem.Text.Trim());
 
                        //   cmd.Parameters.AddWithValue("@Item", model);
                        // cmd.Parameters.AddWithValue("@Price", txtprice.Text.Trim());
                        cmd.Parameters.AddWithValue("@Stock", txtstock.Text.Trim());
 
                        cmd.Parameters.AddWithValue("@Qty_Damaged", txtqty.Text);
                        // cmd.Parameters.AddWithValue("@Sum", txttotalamount.Text.Trim());
                        con.Open();
                        inserted = cmd.ExecuteNonQuery();
 
                        // txtphone.Text = sdr["Phone"].ToString();
                        con.Close();
                        //BindGridreport();
                    }
                }
            }
            if (Convert.ToInt32(quantity) > 0)
            {
                if (Convert.ToInt32(quantity) > availableQuantity)
                {
                    ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('No of Quantity entered is not available in Stock')", true);
                }
                else
                {
                    if (inserted > 0)
                        ScriptManager.RegisterClientScriptBlock(btncustomer, this.GetType(), "alert", "<script>alert('Data Submitted Successfully ... !!')</script>", false);
                }
 
            }
            if (inserted > 0)
            {
                int updatedStock = availableQuantity - quantity;
                using (SqlConnection con = new SqlConnection())
                {
                    con.ConnectionString = str;
                    using (SqlCommand cmd = new SqlCommand())
                    {
                        cmd.CommandType = CommandType.Text;
                        cmd.Connection = con;
                        cmd.CommandText = "UPDATE Store_Table SET Damages= " + customerqty + ", Qty_Remaining = " + updatedStock + " WHERE Item = '" + model + "'";
                        con.Open();
                        inserted = cmd.ExecuteNonQuery();
 
                        con.Close();
                    }
                }
            }
            // BindProductsSells();
        }
    }
}