If you watched them drop it, it was never “found money.”

Found is a word doing an enormous amount of work in that sentence. Watching something leave somebody’s pocket and then calling it found is a decision dressed up as a discovery, and the ten seconds spent saying nothing is the whole of it. No ambiguity was ever available to you, which is precisely why the story has to be told that way.

Say something immediately, because it only gets harder: a tap on the shoulder, a wave, their name if you know it. Where they have already gone, hand it to somebody who works there instead of keeping it. If you cannot catch them, you still know whether you tried.