It was nice of you to try and find him, but should you give the money to the B. Legion? No. without wanting to be rude, you shouldn't have opened it, and it's not your money to give, you should give it back to the post office to deal with.
Are you on the Nextdoor website? I quite often see posts where people are asking if anyone knows of a certain person due to them receiving their post. Might be worth a try.
//and what do you think the PO will do if there is no return address on the envelope?//
Think its RM not PO that TheWinner meant. Answer is probably not much but maybe they have policys for dealing with money/valubles like this, or maybe someones complained that this mail was not delivered
Something similar happened to us a while back, and when all else failed, we took it back to the local sorting office. I think they have a department that deals with stuff like this.
I'm sure it can be traced back to a sorting office in France from the postmark. I remember reading a detective (Morse?) storey where they were trying to trace a sender