It's not the same thing.
We will always be able to pay them the money they win because we take it from the other player.
They have to deposit the money into the game before they can place the bets.
They don't have a chance to win any money from us.. so we're always safe from what you're talking about.
I admire your bravity, but you still have to be extremely careful or you may find out to be wrong with your assumption.
In the easiest case, each game is paid out at once.
Players A, B, and C each put $50 onto "the table". The prize is $100 ($150 minus $50 for the house). Player A wins and B cheats and wins. Both A and B demand you give them $100. What now?
In the regular case, wins are not paid out at once, but go into the customers "account", from which he can eventually "withdraw", i.e. convert your virtual dollars (or credits, points, whatever you call them) to real dollars on the client's real bank account.
Players A, B, and C play 1000 games. Assume chances are exactly the same, everyone wins 333 times (one would have to win 334 times, but forget that...). Each player thus paid 50,000 and won 33,300. You will keep 50,000 profit from these 150,000 and pay the winers from the money tou seized earlier. Easy.
Now unluckily, player A knows a database exploit, so his account gets balanced every time
anyone wins. At the end of the week, B and C ask you to pay out 33,000 each, and A asks for 100,000. That's minus 16,000 for you. What now?
If that's already scary, think about a player gaining root access to your server and creating 5,000 or 10,000 accounts with a few thousand dollars worth of credit in each account. He then withdraws unsuspicious amounts from each. You'll need weeks (or months) to track this back, until then the money will have disappeared in some country where nobody will be able to tell you who the account owner is, or where it has gone.