The module depends on the pos_pin module. Before POS order validation the module checks whether the order contains products with no positive quantity. If it does then a cashier get popup with selection users. A sale is take place if the selected user has group which is specified in the POS config parameter "Negative Order Group". Otherwise the sale is rejected.
Demo: http://runbot.it-projects.info/demo/pos-addons/9.0
HTML Description: https://apps.odoo.com/apps/modules/9.0/pos_product_available_negative/
Usage instructions: doc/index.rst
Changelog: doc/changelog.rst
Tested on Odoo 9.0 9cdc40e3edf2e497c4660c7bb8d544f750b3ef60