If you complain to booking.com support, you should get this back as a credit. I’ve had similar in the pastWhile not a scam, I booked an apartment for Marseille just under two weeks ago via booking.com
I got an email through yesterday (all in French) from the private host who said they've implememted a mandatory 15 euro cleaning fee, not included in the total cost of the reservation.
It's payable in cash to the person who comes to collect the keys on day of departure.
There was no mention of any additional cost at time of booking. Seems a bit wrong to throw this in now. Can't see how they can enforce any such payment