@alisellou If I'm understanding correctly there are ways to do this but if the POS system reduces the stock level directly in your PostGreSQL database then another method of triggering the API would have to be done . You would need to connect both backend DBs as a service in Dreamfactory then write a script that gets data from one and synchronizes with the other.
You can use something like cron or windows task scheduler to make a call that triggers this event script. You can create a cURL script in windows and save it as a .bat file that runs at intervals of your choosing. Hope this helps!