The person asking you politely for free work is probably someone good at sales and manipulating people. In other words, you aren't doing them a favor. They see you as a sucker. You sound like a nice guy, and everyone assumes most people are like themselves. But in business there are tons of wolves in sheeps clothing. The line of people who will take advantage of you will last longer than your life on earth. So you have to learn communication, and learn that you bring value and they should pay what they agreed to pay. Invoice them for the full amount, and don't give them any freebies unless either they complain (even then they need a good reason), or at the very least they have to know that you are trying to do them a favor. If you do free work and they don't even know about it, you have just shot yourself in both feet. You remind me of me. I speak from experience. It's not easy, but it is worth it.