I sort of agree with you that you can only become a billionaire by stealing someone else's money, but in this case I think your argument is kind of bad.

If Yahoo overpaid, it's not the employees of his own company that Cuban took the money from, but rather indirectly from Yahoo's employees (former and current up to that point).

I also have to say, as far as ethics goes, there is enough indirection there that unless you were a dyed in the wool communist you would have problems finding fault with it. The shell game sufficiently blurred where the capital originally came from and it looks more like winning the lottery than exploiting your employees to the people receiving the big paychecks.

Similar things can be said about venture capital recipients. They get money, and obviously it's money that the people who gave it to them did not do sufficient work for them to have gotten it themselves, but the source of that money is so convoluted that it might as well be gambling profits.