I owe a stranger for helping me out.
How did that stranger help you out?
The stranger returned the money I lost
It's not always possible to immediately repay a favor that someone does for you. You should make note of who that stranger is so that you can pay him or her back later. Even if you aren't close and won't likely meet again, something like giving him or her a reward over Venmo for finding your money would be a good idea.
You are right, maybe I will pay him back when I can.
I'm sure they'll appreciate that. While helping someone can be its own reward, its nice to be appreciated for doing so too.