POH says that if fitted, it must be fastened, so if it cannot be fastened, the A/C is unservicable: = You were correct to return, and refuse to fly the aircraft again.
If you saw that the seatbelt had been reported faulty on a previous flight, and there was no evidence of the fault having been addressed: = You shouldn't have flown the aircraft in the first place.
Either way, they did not present you with a servicable aircraft, and so you should get your deposit back.
Just how you can do that depends on the rental agreement and the French legal system.
MJ
Ps. If you are ever asked to pay anything upfront in GA: = Probably best walk away.