You can't prove that. The seller did not have the key, and did not know the amount of money in there. What the seller knew or didn't know is all assumption. What is solid fact is the guy sold a machine with $120 in it, for $40. The seller should be contacted and told.
If the circumstance really is that the seller knew, then it doesn't hurt to tell the seller about it.
If the seller didn't know, then keeping the money is wrong.
Edit: The fact that the OP even had to ask shows that he thinks this deal is questionable. No one posts a thread asking "I bought a game for $50, it was exactly as described and I paid for it. What should I do?"