benevolent flipping
Flipping property in a neighborhood not merely for the purpose of profit, but with a priority of improving the neighborhood. 

One who does this would be a "benevolent flipper." 
Some slumlord offered John $20k for his house, but he sold it to the city for $17k so it would be a rainwater garden instead of yet another North Minneapolis crack house. It's called "benevolent flipping." 

Source: This phrase was used by a guy interviewed on NPR, All Things Considered, in early August, 2008, to describe what he does.  