Maybe call customer support. I am pretty sure prepaid cards can not get overdrawn
EDIT:
How did my transaction go through even though I didn?t have enough money on the card?
In certain instances, because of the way transactions are processed, your transaction could have gone through even though you did not have enough money on your card. As a result, your balance would be negative. Your next reload will be first used to cover the negative balance, and any remaining funds will be available for use. Please refer to your Cardholder Agreement for more information.
Source:
https://www.walmartmoneycard.com/walmart/account/faqs