What is the ruling on a company selling a filter to a customer, and does this fall under deferred-delivery sale (bay' al-salam), or is it selling what one does not possess? And what is the ruling on the company purchasing in this manner from the supplier? And what is the ruling on my work in this company?

The transaction between the company and the customer is closer to a Salām sale, which is permissible. The company's transaction with the supplier is an Istiṣnā‘ (manufacturing contract) which does not require the immediate payment of the capital, and it is also permissible. Therefore, working for the aforementioned company in this field is not problematic.