First, it's piling another crime onto all the other crimes.

Second, any such agreement, by nature of being illegal, would be unenforceable. Even if he has already collected on it, the company would be required to demand the money back if the deal is ever discovered. Yet if he had somehow managed to hide the money out of the company's reach, he would have been in a position to renege on the deal.

TL/DR: don't make illegal deals with criminals who are smart enough to understand the prisoners' dilemma.