I'm not going to give a customer their money when my store doesn't have that much money
What made you not want to give the customer their money?
He made a huge order and overpaid it by a huge amount. He wants change back immediately.
If he made that order, he is entitled to that money and the order. You shouldn't renege on it.
What if I don't feel like giving him the money?
Then you will be rightly branded a thief and the customer will likely sue you for the money. Don't be a jerk and give him what you owe back in change.