I fail to see how the house edge get reduced, even if it was not the topic here.
If the casino allows you to receive the virtual credits before your transaction becomes confirmed, it is possible to cheat the system and end up with more money than you had before.
If they sent you their virtual credits before you officially transacted with them, you could play bunch of games, double them and then withdraw your funds before the first transaction's confirmation.
It's all about the casino's rules. It should not allow you to withdraw anything as long as there are unconfirmed transactions related with you.