At the time of taking the flat you should have made them whitewash the apartment and also replace the Air conditioner if you noticed it was in poor condition. Or, you should have had the condition of the flat and the aircon stated in the contract. I do not know the legal implications but I believe the rule is the flat must be returned in the same condition as it was when it was when it was rented out. The owner will now claim that the flat had been whitewashed and the aircon was brand new when he rented the it out to you. You may not be able to do anything now except swallow the bitter pill.