Easily detected. The H1B visa holder must file a tax return in the country where he or she works. Now a $150k will trigger a very hefty tax which, with all the social securtity taxes, local taxes and whatnot reaches lofty heights. Assume the tax bill comes to $60k. How is that person going to pay the tax bill if only $50k is actually paid out?
Huge and unusual deductions or cooking an income tax statement is fraud which can be automatically fished out if the bureaucrats put their mind to it.
Then there are spot checks and these can include what tax pros call a "root canal audit". This is where the tax inspectors look at actual bank statements and such. Afterwards some of the higher-ups in the company would probably be escorted out in handcuffs.