Finishing Contract - Afraid of being ripped off.

Hey everyone.
 
Like so many other people have experienced similar situations with their hagwons, I am in a position where I feel I am going to be taken advantage of, and unfortunately, lose some of the money I am entitled to.
 
The hagwon I work for isn't terrible, but there are certainly some negative aspects. In the past year I've worked here, there have been numerous instances (which I won't mention in aim of making this message shorter) where they have taken advantage of foreign teachers. Hopefully you can understand the kind of situation I am implying.
 
Anyway, the reason I am writing this letter, is because I am finishing my contract in December. Recently, even though my contract says the school will provide round airfare, my school supervisor (who is a very lewd emotionally unstable person thus undeniably hard to deal with) said that the school will only provide (undisclosed amount) as a flight allowance. However, nowhere in my contract does it stat I will be provided with a predetermined amount of money for my flight. It simply says it is the schools responsibility to provide round airfare for foreign teachers.
 
So, I am annoyed by the situation. I've not spoken with them directly about this issue yet, (again, dealing with him is very unpleasant.), but do I have any leverage if they try to skimp on the flight? The cheapest ticket I can find to my home is approximately 1.2million WON, and they had no problem paying that for me to get here.
 
Also, I am afraid about being ripped off on my bonus. They deducted other teachers who've successfully completed their contracts, with unindentifed 'bills', even though our bills are very low and we settle them up monthly. He lost maybe 500,000 in expenses which they didn't explain, and manipulated their justification in a very belittling manner.
 
So, what do I do? If they try to steal money that is mine from completing this contract well, do I have any leverage? Any advice would be greatly appreciated.