I don't do this because Parsons and Williams both have player options next year. That means that the C's could conceivably give up a first round pick to pay D-Will another year, while Parsons walks.
If as part of the deal Parsons picked up his player option for next year then I would do it.
This early, the Mavericks don't look to be as bad as I expected them too. Wes will continue to improve after his injury last year and Parsons should come back and get better as the season moves along. I could easily see this team making the playoffs and would not hesitate to trade a pick in the mid to late teens for Parsons (provided he picks up his player option as part of the deal)