Did he/his father say that he lost the finger because he broke the promise? Did he tell the story as a morality lesson? yes on both counts!
***** SPOILER *****
Stu's father, Frank, lost his pinky finger in an accident while working in a sawmill that supplied a paper mill. He told the young Stu that he had lost it due to reneging on a pinky swear. Stu, from that point onwards, cultivated a habit of doing what he promised to do!
Congratulions, Balin, on the solve, and thank you everyone else for playing and asking some great questions.