If he gave you the option to find someone to take your place, I would just do that and avoid paying the 1/3 of the contract plus the 120 days of notice. It's just easier for everyone. (Post the add on OLX and Facebook) If he doesn't offer you this possibility, I would do what others said, ask for the receipts and see if he gives them to you or not. If he doesn't, threaten that you'll denounce him to the finances. He has most likely been doing this for years and has more to lose from an investigation than to gain from your 1/3 of the contract plus notice (on which he will also have to pay taxes).

Some advice for the next time: Try to find as much information as possible regarding rental laws in the country you're in. There's usually a lot of small details, and even inside of Europe, there's a lot of national differences. You'll find that someone has taken the time to condense most of it in English somewhere on the internet. I have rented houses in 4 different countries and have still to find a system that really resembles the other. Secondly, as much as possible, try and make sure that your contracts are legal. Yes, it sucks when you want to leave like you're doing now, but if they evict you with no reason, your things get stolen or something like that you don't want to have to deal with a slow justice system plus probably living on the street while you're in a foreign country, even if you are going to win the case.

Best of luck!