was your deposit kept and cashed by the agency itself or by the apartment owner? I suppose you have already tried to write to the agency. In France, to be able to keep the security deposit, they have to send you the bills or repair quotation (after damage) as a proof. Here they absolutely need to send you the bill. True that from a distance, things are difficult. I would say you should send the agency (or the owner if you signed the contract directly with the owner) a registered letter asking them to provide you with proof that you owe that money or else refund you and remind them that the French law requires the person who illegally keeps the deposit to pay also the legal interest rate from your date of departure. If they see you know the law, they might proceed?..... good luck