Need different reference series for Direct Invoce and Delayed Invoice

I am trying to implement following :

In my setup of FA two type of sales occur Direct and Delayed, what i need is separate reference series for direct invoice(cash sales) and Delayed Invoice(credit sale). I tried this by adding following at line 158 to cart_class.inc file but my problem is not yet solved

         #156                    if (!is_date_in_fiscalyear($this->document_date))
    #157                $this->document_date = end_fiscalyear();
    #158     $this->reference = references::get_next($this->trans_type)+1000000; // added 1000000 to the number
    #159            if ($type != 30) // Added 2.1 Joe Hunt 2008-11-12
    #160            {
    #161                $dim = get_company_pref('use_dimension');
    #162                if ($dim > 0)
    #163                {
    #164                    if ($this->customer_id == '')

my idea was to have the reference number to be incremented by 1000000 if it was trans_type 10. However it is happening for every type of transaction.

Any help on how i can do this ?

I would also appreciate other ideas too. What is important to me is all my credit sales are having references in serial with respect to starting number setup in forms setup. Cash sales may or may not have reference.

thanks