I'm going to sleep with a woman I've been volunteering to help
Why would you sleep with her of all people?
I want to get something from her because she took advantage of my charity
You shouldn't take advantage of this girl just to have sex with them. You could really damage this person's life by doing this.
It's not abuse. I barely know her and she was the one who wanted to sleep with me.
Okay, if she is truly interested in you, then that might change the situation.  However, if she is relying on you for help, it could still be against the rules for you to have intimate relations with her, and could possibly be illegal.  Only when she is fully able to make her own independent decisions should you be willing to have sexual relations.