Is the amount of money taken by the questioner from a stranger years ago considered a debt that must be repaid, taking into account the smallness of the amount and the passage of this long period?

It is highly probable that the amount paid was a gift, not a loan, which is why he did not come to collect it. His kindness should be reciprocated with kindness, either through a gift or a prayer. Nevertheless, you can go to him, thank him for his past stance, and clarify that the amount is still a debt. If he absolves it, then so be it; otherwise, pay it to him.