What is fair and what is not in the work place?

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A colleague of mine was transferred from the administrative department to the sales department at the request for our employer. She had been really working hard--with her going to the office in the morning and then doing sales calls in the evening until dawn since most of our clients are based in the US--without any overtime pay or any other incentives. She has a basic salary and gets commission for every deal she closes. For about a year, she was able to bring in 6 or more new accounts/advertisers (our company is a publisher of a magazine that is distributed all over the Asia Pacific region), however, our employer criticizes her for not getting specific companies (big corporations) to advertise with us. Although the reports shows that she had been doing a great job, our employer still criticize her sales standing. Then one day, our employer informed our colleague that she will be lowering her basic pay to the industry's minimum pay. Then aside from this, she is blamed for every mistakes committed by one of our colleagues from the administrative department. And then after 3 months, my colleague decided to quit. She turned over her work properly and she expected to receive her back pay and the commission she got from the deals she worked on. After 2 months, our employer released her basic pay but did not release her commission. The reason she gave was that she invested a lot on our colleague because she let her travel and do sales calls to clients based abroad. She said that it is not enough to cover the expenses she paid for the travels. Even before our colleague accepted the post, there was no contract signed for any travel bonds. Moreover, she was never sent to trainings or seminars to learn how to sell. She learned and develop the skills on her own. She did not deserve the treatment she got from our employer but she never filed a case against our employer but I am urging her to do so. What case should she file? Do you think she has a case?