How do I create a fix as a result of the save system...

Within my game I have included a way for players to gamble their earnings from the game however this opens up the ability to then save and then gamble and if they lose they could just load the old save and get their money back... Is there any way to fix this.

One method i have would be if the save system let me keep a variable across all saves so this will save the amount of money lost between saves and then deduct that when the user tries to load an old save.... However I don't know how to do that....

Perhaps you can have the game automatically save the player's money stat right after the player's done gambling, that way reloading should still keep his post-gambling results.