Well it depends. If it was drug money I'd probably be like Llewelyn Moss in No Country for Old Men (but I wouldn't take a jug of water back to some hombre that got wounded in the shoot-out). But if it belonged to an honest citizen I'd give it back and wouldn't take a reward. I might accept a free home cooked chicken fried steak dinner though.
I would have the dilemma of returning and would try to contact and see if the person who lost it was honest. I would have to question why it was in a backpack and who loses 3300. To me my first thought is that money was not obtained legitimately most likely.
If I knew the owner or there was some type of marking to trace the owner I would return. If not, I would consider it a blessing. However if I were homeless, I am not sure what I would do.