Labour Law.
I think you are right about they can treat the crews the way they want in the gulf. I know some ex SIA pilots are taking action in the UK against SIA for breaking the EU law against them. They wouldnt get away with it in Singapore. I also know a captain who used to work for a tuboprop operator in AUH and after they went to see the judge in AUH at labour court and announced everything was settled when he left. When he went to pick up his passport he was told that he had to reimburse the company a big chunk of his last month salary since they decided he would not get his bonus they already paid him. He paid back just to get out of there and never come back.
I was told the best to do was to tell the judge that you havent got all settlements and I you could please have his number in case they wouldnt honour the agreement or the law.
I dont know if you still have to go to labour court when you leave.
But this is how it was.