So I take it you're aware of the concept of buying good feelings, and true positive impact separately? http://lesswrong.com/lw/6z/purchase_fuzzies_and_utilons_sepa...

Ah, and here I was looking for an example of LessWrong being neoliberal in outlook!

You can't presume that the exploited-away surplus-value or negative externalities from your high-paying-donate-to-charity job actually exceed the good you're doing by donating to charity with the proceeds of your work. Warm fuzzies are, at the very least, a reasonable guide to doing work that's relatively sure to be relatively purer good.