Is my project activity permissible or impermissible (halal or haram), knowing that I work in telemarketing for a company that sells solar panels, and the customer has the freedom to choose between different payment systems, including cash purchase, leasing, and interest-based loans, while I am not involved in determining the payment methods nor do I own the product?

If the company's activity is permissible, which is selling solar panels, then there is no harm in promoting them and taking a commission, especially if the company provides permissible payment methods. The existence of impermissible payment methods does not affect the permissibility of your work; the responsibility lies with the company and the customer who chooses the forbidden method, and everyone is responsible for their own actions. The mere illusion of assisting in an impermissible transaction does not have an effect because it is indirect and unintended.