Is it possible to modify the basket in OpenCart for the specifics of trading?

Question

CMS

Hello, there is a site on opencart 1.5.5.1
How can I implement it so that goods that are not available (with asterisks) are not included in the total order amount. That is, a person threw in a basket of goods for 5000 rubles, of which only 2000 rubles are in stock. And he can only pay this amount in the future. But in the order, I need to come and the goods that he did not pay for are not available. Thanks for answers.

Answer 1

/catalog/controller/checkout/cart.php
in this file this logic.
I don’t remember exactly, but it seems that syste/library/cart.php is responsible for the total ->getSubTotal();