Is the friend of the questioner, who owns a company that produces electricity from solar energy, sinful if the client takes a non-interest-bearing loan from an interest-based bank to pay for the service, and does the ruling differ if the loan is interest-bearing?

If the loan is non-usurious, there is no objection to dealing with the customer and accepting payment from it. However, if the loan is usurious, it is not permissible to entice the customer to it, due to the Almighty's saying: "And do not cooperate in sin and aggression." But if he pays the price from it, the transaction is valid.