Marci, it's ok. I'm sure the lady will keep an eye on him and maybe even manage to get him in the car. Just check the newspapers tomorrow to see if anyone placed an ad. Also if you have an animal shelter you could inform them where you saw the dog in case the owners call them.
We saw a black (looked lab mix) today, but couldn't stop to pick it up because we had a car full of grocery's.
It's ok though, someone else will get him/her I'm sure. So don't beat yourself up ok!!