Look at it this way. The guy already thinks he's lost the money and all his info. If it makes it to him, he'll be very happy. If not, he'll never know the difference. (I hope I worded that correctly....did not want it to sound callous) You did your good deed....now it's up to the postal carriers etc. to continue (though they'll never know what's inside)
Either way, it was a great deed. Karma will be on your side
That was your only choice no matter what. You knew it wasn't yours, you knew whose it was. In the mail it goes, doesn't require a phone call, doesn't need a return addy, the Guy Upstairs knows who found it.
He that would make his own liberty secure, must guard even his enemy from oppression; for if he violates this duty, he establishes a precedent that will reach to himself.
Thomas Paine