If you know who the issuer is and have the CVV, just go to their web site. Otherwise, you are out of luck.
You can buy anything you want, and swipe the card with the partial balance first when you’re ready to pay. Let’s say you buy $20 of things, and have $10 left on the card. You’d pay $10 on the card, and $10 by another means.
Of course, if you really want to zero the balance, perhaps buy that amount of gasoline and be done with it?
For gas you’d probably have to go to the cashier. The pay at the pump method does a pre-authorization of a certain amount. If your card has less than that, the authorization would fail. You might be able to go to the cashier and say to put $5.21 and that’s all that you’d need to have on the card.
I’ve had luck using these money cards at the self checkout. It takes whatever balance is on the card and then prompts me to enter another payment method for the remaining balance.