it will usually be a lot more effective to put in some more time at work, and give away some of the resulting income.
I'm paid salary. I'm of the opinion that it is usually the self-absorbed that think that after they leave the office they are still worth what their employer pays them.
Making it a requirement that your time donation be within your professional skillset is saying, "such work is beneath me". I'm not above ladling a bit of soup into a bowl or picking up the dog shit while at the animal shelter.