This appears to be a riddle, and the answer is a poor man (or simply the poor / a pauper).
The reasoning: A rich person already has wealth, so they don’t need to closely watch or observe how it changes or grows — it’s stable or increasing without much effort. But a poor person must constantly observe and track even small changes in wealth (a few dollars gained or — more often — lost) because it directly affects their survival, bills, food, etc. Every tiny shift matters dramatically to them.
A classic variant phrasing of this riddle is something like: “The rich need me, the poor have me, if you eat me you’ll die — what am I?” (Answer: nothing). But your version focuses specifically on observation of wealth dynamics, pointing to the same ironic truth: the poor are the ones who truly watch how wealth changes (or fails to change) on a daily basis. The wealthy often don’t have to.
So, “him who observes how wealth changes” = a poor man.