I believe both in Karma and in God...they both repay.
I have also found most recently that being nice to people you would rather slap in the face is not for their benefit, but your own. It releases something inside you, and their slinging arrows go right past you. I try to practice this in my personal life, but lack on the boards lol. If you live with unforgiveness in your heart, it actually hurts you rather than the one you are angry with. It pollutes your mind just as jealousy, anger, rage, and envy do and that stops you from receiving the blessings you feel you deserve.
When you do good to others who hurt you, it forces God or the Universe for some to be able to operate. When you are too busy trying to repay your own enemies you block what could be done by a higher power...that power can usually do a better job of repaying than we ever could.
I tithe out of my need and in faith...meaning BEFORE my bills are paid, and it has never once left me without. When I can't afford to give, I give even more! If you give out of your need meaning what hurts to give up, you will be rewarded with more than you can possibly imagine. Keep in mind the "where" or "what" you give to does not matter. It's the compassion behind your giving that makes the difference. Just because a preacher is robbing people does not mean "your gift" will not be repaid.