Need cash fast?

I opened a credit card that has absolutely no transaction fees for balance transfers, cash advances or foreign transactions. I understand that there is no grace period for interest on cash advances. Am I correct in thinking that if my card has a negative balance, I could take a cash advance and pay no interest charges? Wouldn't they be calculating the interest on a negative balance?
When I refer to a negative balance, I'm going to make a payment in excess of the amount due on the credit card. The negative balance is the amount the credit card company would have to refund me to return me to a zero balance.