Earlier quoted context omitted.
Those aren't "extra dollar bills in your wallet" any more than a tax refund is. You still paid, you just paid less. In the case where you didn't go back, you'd have had even more "extra dollar bills in your wallet." An (absolute, not relative) reward for good behavior — i.e. a positive-ROI outcome for doing the right thing, vs. not doing the right thing — would be, for example, if you came back and told them you didn…
Is it accurate to say your argument is that stealing something costs less than what happened? Because I’m having trouble understanding your point.
To be clear, theft isn't theft (legally or ethically) without mens rea — if you accidentally walk out of a store with something, you haven't done anything wrong.
And if you later realize you now have possession of the thing, that doesn't mean you're suddenly guilty of theft, either. Theft is an act requiring mens rea, not a state of someone who possesses something that was at some point illegitimately dispossessed from its owner. (If the latter was true, then most people who buy anything from pawn shops would be guilty of theft, because those things were often stolen from someone at some point.)
You might, separately, see there being positive ethical value in returning an item to its owner; but this ethical value should be considered independently from the ethical cost of having stolen (or not stolen) the thing. If you think there's positive ethical value in returning things to their owners, then you'd likely feel an ethical impoetus to e.g. "steal" stolen bicycles from those piles you see on the front yards of flop houses, and bring them back to the people who own them. (Even though those people have probably bought new bicycles already, and won't know what to do with the returned ones. Even though the stolen pile of bicycles is likely just a pile of rust due to lack of maintenance.)