How is it greedy? You buy a house, someone builds a runway that devalues your property, surely you have a right to compensation. You may have to install double glazing to keep the noise out and it could effect your quality of life but it's through no fault of your own.
The cash is a token recognition of the fact that the second runway has had an impact on your community and Manchester Airport agree as well, because they're paying out even though there's no legal requirement for them to do so. It's corporate responsibility!