Seriously how is it anything less than a huge blow to this team given it finally had a valuable trade chip that it could cash in and then have that yanked from them. LA wasn't going to give up much to begin with and now they know it is either them or no where. The Cubs also lose a chance on help setting the market in this new trade market in a very positive way for sellers.
It makes 0 sense for him to veto the trade. He'll be going to a team that's probably headed to the playoffs for the second straight year. He's going to be a FA at the end of the year, and if he plays a vital role in the Braves reaching the postseason, it's only going to heighten his perceived value. It will also heighten the chances of him getting an extension from Atlanta before the end of the season... or at least this winter. It looks like both Hamels and Greinke are going to be taken off the market... and if the Brewers extend Greinke, I'd be surprised if they don't do the same with Marcum. This would mean that Dempster suddenly becomes one of the top 3-4 SPs available... and now has a recent record of helping a team to the playoffs.
It's a win-win-win situation. We get a building block for the rotation of the future... and possibly another arm. The Braves get a vet pitcher that's having an incredible season and whose leadership skills can provide a boost overall to their team. Dempster gets the chance to showcase himself on the big stage as a solid FA acquisition of this winter, thus having his value jump through the roof.