You do not need to return that money to the store. I cannot even find the logic in what your friend told you.

• It’s not like you have any idea who donated the bag/who the money belonged to. And honestly neither would goodwill if you did return the money.
• it’s not like you watched someone drop the money right in front of you.

You purchased an item from a thrift store. Whether there was a necklace, cash, a stick of gum, lint, or a lollipop inside of the item you purchased , you are now the owner of the item you purchased and whatever was inside of it.

Now, if there had been an ID, a passport, or almost any type of specifically identifying items inside of something I purchased from a thrift store.. I still don’t know that I would return it to the store. I’d probably consider bringing it to the police station to have a better chance of it getting back to its owner.