Sorry my friend, but this is France. Everyone wants to rip you off. Being a johney foreigner does not help.
I would never ever ever buy a flat to rent out for holiday rentals. You are just asking for trouble in France…..which you have just learnt the hard way.
Sell up, or put it back into order and enjoy your second home.
Dont go down the legal route…..it will cost more than the flat is worth. Put it down to experience.
Good luck.