Is working for a contracting company that delivers natural gas to homes—a company whose owner contracts with citizens either in cash or in installments through a usurious bank—considered unlawful or a form of cooperation in usury, knowing that the company does not interfere with the client's method of contracting with the owner company? And is there any sin upon me if one of the employees incites citizens to contract in installments without my knowledge?

There is no objection to your work in installing solar panels, and this is not considered assistance in usury, because loan contracts do not depend on your installation. You are permitted to take your dues from the company because its money is not entirely forbidden. As for encouraging citizens to enter into contracts, it is permissible if it is a general encouragement without referring to the forbidden installment system; otherwise, it is not permissible because it is an invitation to sin. You bear no sin for what occurred without your knowledge.