I have this happen, and will tell you my temp workaround until it is properly fixed. If I ask them “Can I have some equipment before leaving the shelter” before going to do a quest or if you chose any other option other than “I need no payment” once completing a quest I get the same problem where it says I owe them a certain amount of money when trading. If you do chose “I need no payment” once completing their quest, they will give you whatever you want and teach you whatever they know with a 100% chance.
Also if you get them to come with you before completing the quest, and have them teach you anything while you have a quest active from them, it cancels that quest, so don’t ask unless you want a quest in your mission log to stay there for the rest of your game without being able to complete it. Maybe there is a way to remove it, but I haven’t looked into it. Solusphere told me about that one when I made a NPC bug thread last week about NPCs training skills. Here’s the link if your interested… not much there i didn’t mention here though. http://smf.cataclysmdda.com/index.php?topic=14846.0