MAVS NEWS:

[Mavs2021]

The Mavs are now $12,097,601 below the tax line. Which opens up the MLE that they have remaining, $12,044,000. Coincidence?

Obviously not. Klay had his mind set on Miami. Dallas could not find a satisfying player deal. So they told Klay that if he wanted to go there, he had to give up this exact amount of money. 

Then Klay probably told Miami, who theoretically had this amount of money left. In the end Klay sacrificed 3-4M out of his own pocket and the deal was reached.