Is it haram for me to do work for GM’s company during ABC Company’s working hours?

I am working in a company (ABC) who is GM hired me directly as Receivable accountant, GM is assigned by the owner and fully responsible for all matters of the company. I am doing my job and fulfilling all the responsibilities as per my job description. At the same time GM owned one company which is situated in the same building where I am working, GM hired me as part time accountant in his own company and pays me some amount of money for that. My Question is that some times GM assigned me to do the job for his own company during the working hours of ABC Company, and I am doing the work for him during the working timing of ABC Company. One of my friend told me that you cannot do the work for others because ABC company pays me a salary and I have labour contract with ABC company according to that I should fulfill my working hours for ABC company. Kindly advice me is that haram for me and if this is haram what will be kaffarah, because I am doing this since 3 years.