According to dallasbasketball.com, the Mavs FO is trying to save the MLE for someone else (Maggette, Posey, etc.) and will instead try to obtain Diop through a sign-and-trade for assets such as expiring contracts (Eddie Jones). See, Donnie's trying to be creative.
Supposedly, the Mavs have a great deal of interest in Posey, who played a huge supporting role for the Miami and Boston championship squads.
The Mavs' summer dealings may not be done yet.