Draftees contracts in the NBA are not negotiable, BUT they don't have to be signed. Even in the first round.
If a team drafts a player in the first round, then grabs a big name free-agent and realize they have no more cap-space, they can simply decide not to sign the rookie's contract. At this point, he cannot play for the team (or any other NBA team since that team has his draft rights) so he can only play overseas.
"Air is beautiful, yet you cannot see it. It's soft, yet you cannot touch it. Air is a little like my brain." - Jean-Claude Van Damme