I also returned a wallet and was offered a small reward but refused it.
My children were raised to do things like that without expecting any reward. That's why my son kept refusing in the incidence that I recounted above. But he finally accepted when the wallet's owner kept insisting. Perhaps he saw that my son was a young guy who could use some money.
I was actually inside the house, while my son went out to give the wallet back as the guy drove up. When my son went inside and told me the story, he said he accepted because the other guy was very insistent, and used the word "loaded".
I told my son that this guy was so happy to get his wallet back, and wanted to show his appreciation. And my son, by accepting the reward,
gave the owner the chance to give thanks. So, I explained to my son that it was quite all-right.
My son said that he felt bad now because the $40 was too much. If my son was quicker, he could have said "No, I cannot take your $40, but will gladly accept $20". I was also surprised that the reward was that much.
If it were me, I would have replied jokingly "But how do you know I am not more loaded than you are?".