This thing doesn't make much sense, are your members expected to have calculator next to them when gambling? And somehow i doubt that casinos with "mechanic" like that will keep going in the long run. There must be some kind of protection for users, and it has to be user friendly to keep retention high.

I worked in gambling industry for years and saw my share of mistakes/similar issues, and what proved to be the smartest  move was to man up , pay the win (in some cases casino and gambler found some middle ground and from what i noticed, @yahoo62278 suggest that as well) , and make sure that kind of thing doesn't happen.

Think it this way, it could have been even worse for you, someone could  bet much higher amount, and then it would be even bigger issue for you. Keep in mind, sorting these kind of things in customer favor is also good advertisement, and way to go in this business in the long run.