I want to reduce my stock when bill is generated/saved

I am using this method to save customer data and sales as per ID. Now i want to reduce my stock when bill is saved or generated.

public function addBill(Request $request)
{
    // $this->validate($request,[
    //     'cus_id'=>'required'
    // ]);
    $customer = new Customer;
    $customer->cus_id_no = $request->cus_id_no;
    $customer->name = $request->name;
    $customer->p_number = $request->p_number;
    $customer->address = $request->address;
    $customer->cus_type = $request->cus_type;
    $customer->total = $request->total;
    $customer->tax = $request->tax;
    $customer->g_total = $request->g_total;
    $customer->paid = $request->paid;
    $customer->remaining = $request->g_total - $request->paid;

    if($customer->save()){
        $id=$customer->id;
        foreach($request->productname as $key=>$v)
        {
            $data = array('cus_id'=>$id,
                          'pro_id'=>$v,
                          'actual_cost'=>$request->actual_cost[$key],
                          'qty'=>$request->qty[$key],
                          'price'=>$request->price[$key],
                          'dis'=>$request->dis[$key],
                          'amount'=>$request->amount[$key]
                        );
        Sale::insert($data);
    }
}

Stock is my Model name and stocks is my table name.

via Chebli Mohamed