Do NOT pay anything until you purchase the property in Land Registry Office. The contract you mention does not mean much in Turkish Law system. You MUST purchase the real estate in Land Registery Office, otherwise you can not claim ownership or sue the other guy to pass the ownership even if you have a contract with witnesses. The best you can do is to sue him to get your money back depending on your contact but this can be a problem considering the fact that you are abroad.

So safest way for you is to not to pay anything at the moment and ask for a official sale in the registery office and you can pay at the moment they signed the official papers in the land registery office.