This morning I dropped Orlando off at the vet for a test. While I was waiting for paperwork, I noticed the only other client there, an elderly woman who was dropping off a dog. I watched her pay a $300+ deposit in cash (all in tens), and then heard her ask if there was a nearby coffee shop where she could wait -- one within walking distance, as she can't drive any more and had taken the bus there. The person who was helping her indicated a plaza about half a mile down the road, and the woman asked if there was a bus stop there. (There is one in front of the vet's office.)
I told her I was going that way and would be happy to drop her off if she wanted. (She'd have to find her own way back later.) She accepted. This was no imposition on me; I was driving right past there. But she made a big fuss, and as she got out of the car she pushed a $10 bill on the dash and refused my rather insistent pleas to keep it. I was somewhat horrified. Appearances can be deceiving, but I judged that she needed it way more than I do. And anyway, $10 for a half-mile ride and a little conversation? I considered it likely that she didn't have a $1 or $5 bill and so reached for what she had.
This led to a dilemma and an interesting discussion in the Mi Yodeya chat room. I was certainly not going to keep the money. The default answer is to give it to charity, but I wondered if there were some way I could return it to her without causing problems. I considered asking the vet to "discover" an "error" in her bill for me, but it was pointed out that I'd essentially be stealing her mitzvah (she presumably thought she was doing one), a position I hadn't considered. I also wondered whether she would put two and two (or ten and ten) together, figure out what had happened, and be offended (causing offense would be bad). I would make the connection, I'm pretty sure, but apparently I am not normal. :-) (I don't think I'd be offended, though.)
I considered asking the vet to find a larger "error" in her bill so I could help her anonymously. But in the end I decided that this kind of sneakiness isn't appropriate. So when I picked Orlando up after work, I dropped the $10 bill into the donation jar for an animal-welfare organization. It seemed fitting.