Cuban has sold the Mavs to Adelson-Dumont Family

[WillE]

I wonder if that 20% will be valued differently at a later date?

I cannot imagine Cuban accepting anything else.
He will have continued to bet on "himself" resp. the Mavs and their steadily increasing value. They may just have already defined the evaluation criteria and methods, but I assume that the whole point of this option is to create an incentive for Cuban to hand over the business smoothly and continue to steer its development in the right direction.