...Nothing I can do about it now...
Seriously? This is the best you can do? If it were me, and I finally had an epiphany, I would probably go back and give the bartender the $100 back saying I dogged it and wanted to make it right. However, that is just me.
Under those conditions the bartender might not be willing to accept the money thinking that if you dogged it then that was a consequence of gambling.
IMO the best way to return the money and walk away with good consciences would be to play your buddy again for $100 with your own money and beat him out of $200 and give the bartender $100 as his half for having faith on you the last time. This way he will accept the money all will be well and let this be a lesson in morality for you.