Mark Cuban tried throwing money at Dallas
First of all, Cuban's the legal owner, thus the league rules apply to him. He simply can't toss cash or non-monetary bonuses at the players w/o it affecting the cap.
On the other hand, what prevents a random stranger, going up to Leon Powe this past summer, and handing him a both a check for a million dollars plus a legal 1099 misc statement and then telling him that there's more of that to come, if he doesn't sign to the Cavs for more than one season

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I hope you're beginning to catch my drift.