Dam younger generation has no honor or dignity
I don't necessarily agree with this statement; however, there seems to really be no honor among thieves.
I'll state it again, for those who missed it: the guy wasn't showing off if he accepted twice. The guy wasn't trying to do a RWT if he didn't check a few times to make sure he had the right guy. I'm guessing that he sincerely clicked the wrong person (if you're at a bank that's easy enough to do) and thought he was giving it to his friend. When his friend told him that he traded the wrong person, or when he checked his trade log, I'm sure he realized the mistake he had made.
For most people on this website, 40mil is chump change, as next to everyone here bots, exploits, and knows the secrets to money making. For the majority of RuneScape, 40mil is no small amount. Up until I joined Sythe actually, I had 20mil to my name. I started learning the secrets to money making, making and helping friends, and working harder for a little bit of cash (and getting a very generous gift from Dragon Dyce).
The point I'm trying to make is that the guy gave you a large sum of money that he probably had to work for. I would have, in good faith, given it back. Many people don't believe in karma. Had you given it back to him and then posted about your honorable deed, I feel as though you would have gotten much more positive reputation around here than posting about keeping it.
I know I certainly would have hailed your act of chivalry had you given it back.