I can't believe my luck! I just found $300 in the back of an unused picture frame in a box of junk an acquaintance left behind to donate to a thrift store.
Wow that's a lot. What do you plan to do with it?
I think I'm going to keep it.
Even if your friend forgot that money in the picture frame it's still their money. You have no right to take it, you should give it back to them.
Nah, I'm going to use that money to buy clothes.
You should consider calling them to let them know. They might give you a finders fee. How will you feel if they come looking for the money after you've spent it.