[quote name='DuelLadyS']That was nice of him. I mean, think about it- the shirt (and most likely other goods) were dontated to goodwill. the goodwill people priced it, shipped it, stocked it, and sold it. Technically, doesn't that make it (arguably) legally his?[/QUOTE]
Actually, I am in law school, and I have my Property exam this coming Monday, so I may be able to shed a little light on this. Laws vary from state to state, but generally speaking the concept of finders is that if you are a finder, you have title to what you find above everyone except the true owner (the original owner of the $2000). The shirt was donated to the Goodwill store, meaning that title of the shirt was transfered to the store, but not the money hidden inside it (it was "mislaid" by the original owner). Since you in good faith bought the shirt (bought it legally), title to the shirt has transferred to you, and since you found the money, unless the original owner finds out about it and can prove that it was theirs, it belongs to you.
For those of you considering law school one day, this is the kind of stuff that you do.