I would have read this short story around 1970 to 1990. I'm pretty sure I read it in an anthology like Galaxy or F&SF as that's all I read back then. I keep thinking it was titled "Notes from the field" but that must be an error. The story was mostly in the form of reports from this investigator back to his supervisor.
The investigator is trying to get a large amount of money to a man who desperately avoids him even though his family lives in poverty. The investigator follows him and finally discovers the man (whose father was a drunken musician, and whose mother raised him religiously) and forces himself to play piano in a honky tonk bar to make ends meet. Story ends with something like "he was his father's son".