If you go to the Belediye offices they will confirm how much you need to pay. You did that and were told 480 YTL. I am not the defender of Turkish fair play, indeed I could tell tales of corruption that would quite possibly horrify you. Now, if the story was that the clerk didn't tell you about the 25% discount for early payment, I would believe it. I would believe that clerks don't tell non-Turkish speaking people about this. I would suspect they might well take your 480 YTL, stick 360 in the till and 120 in their pocket. It's wrong but it wouldn't surprise me one tiny bit. Not being able to speak the language makes you weak. Understanding what needs to be paid is YOUR responsibility, not theirs. You have evidence to back up your ludicrous funding scandal theory here there and everywhere - comparisons that cement your beliefs even more; but your evidence is total rubbish. The man who pays the same as you with the "huge duplex" technically has the same as you really, a one floor apartment with a roof space. The people who have a massive apartment who pay less than you probably have a much lower value on their tapu.
You see for me, there is no story over there you can tell me about corrupt people that I could never believe. The problem is you have concocted a conspiracy theory that involves the entire Belediye budget being approved and passed based on mathematics that charge only foreigners - and that is something you must have dreamt up in cloud cuckoo land.
