Why is the contract between Timberwolves and joe smith illegal?

In the distant 1998, under the persuasion of the famous sports agent Fagan, the Warriors traded Smith to the 76ers and transferred him to the Minnesota Timberwolves in the summer of 1998 (it seems that his hands were upside down), and then the most famous "salary cap scandal" occurred in NBA history. At that time, Fagan signed a fake contract with the Timberwolves far below Smith's market price (US$ 6,543,800+750,000 for two years), and then secretly asked the Timberwolves to promise Smith a long-term maximum wage contract (US$ 86 million for seven years) in order to collect huge labor fees from it. As a result, the incident was discovered by NBA officials, and the Timberwolves were severely punished, deprived of the first round draft pick in the next five seasons and fined heavily. Joe smith was also ordered to terminate the contract and become a free man, and both sides lost. Smith still seems to be Fagan's agent. . . . . .

To put it simply, at that time, the Timberwolves had no salary space (you know the big contract of KG), and Smith couldn't find a good owner in the market, so he signed a low-paying contract with the Timberwolves, and then the Timberwolves and Smith privately agreed on how much data to play, and there would be a big contract after three years (because there would be bird rights after three years). It is said that this agreement does not exist in the league, but people are verbal, while the Timberwolves are written. ...