Add custom css file after plugin css with WordPress Child Theme functions.php

The original stylesheet is enqueued as openpos.bill.bootstrap

wp_enqueue_style('openpos.bill.bootstrap', OPENPOS_URL.'/assets/css/bootstrap.css') // load my custom css after this one

But you’ve set openpos.bill.style as the dependency:

wp_enqueue_style('bill-screen-style', get_stylesheet_directory_uri() . '/include-parts-from-functions-php-file/frontend-related/plugins/openpos-plugin/pos-app/css/bill-screen-style.css',array('openpos.bill.style'));

These need to be the same.