I think you are having problem to understand how this script actually works or how it was built to work. Yes you can get it customizes more by any PHP developer. I am going to explain a little how this script works so you can understand.
1) You add a product (Its available stock is 0)
2) You set the Selling price of Product and you set different prices by going to Price levels so 5 groups of customers can have different 5 prices based on priority or discount you want to give specefic level of customers.
3) You add vendors, their Balance is 0 and no invoice is added.
4) You purchase something from vendors and add purchase invoice, If you pay it via cash , Vendor balance is 0 , if you Select credit vendor balance is invoice's total. Which you can later go to payments and pay partially or full amount. So if a vendor have 3 invoices Invoice 1: 200, Invoice 2: 300, Invoice 3: 500 and you paid 3rd invoice by cash and first two by credit you will see vendor balance 500. If you go to payment and add partial payment of 250 RS the vendors remaining balance would be 250 RS.
5) Once you added invoices, the quantity of products is added into your stock and relates to your warehouse not vendor. Any sale happen cash or credit you will see the balance of customer. If Customer A bought 3 times, Invoice 1: 300, Invoice 2: 200, Invoice 3: 100 and he paid 3rd invoice as cash and first two as credit, The balance of client or customer would be 500, and you can add receivings full or partial later by going to recevings.
Let me know if you have further questions.