That's great. It might even be a legal rental then, unless all of the payments are still in cash -- which would confirm that it is illegal. Otherwise you could have made a bank transfer in the first place.

Payment with cash may point towards an illegal rental but confirmation it is not. I can rent my apartment for up to 4 months a year, accept payment in any form I like, including cash, and the rental is completely legitimate.

French residents are actually allowed to pay cash for legal transactions up to 3000€. Non residents can pay in cash up to 15,000€ for legal transactions.

That is something very much different from the creditor <b>demanding</b> payment in cash. Any legal merchant is not only perfectly happy to receive payments by bank transfer, check or credit card but obviously very much prefers it since it leaves a trace of the transaction and eliminates any risk of counterfeit banknotes, or being robbed on their way to the bank.