Trying to buy vBuck from Fortnite from a family account

I am trying to buy $24.99 of vBucks from fortnite for my son.

He initiated the purchase and it said it will ask permission. I get the notification that he is asking permission so I click Accept. Then it says I need to add money to the account. The account has $35, but it keeps asking me to add more, even though it
clearly says he is asking to spend $24.99. I figure there is tax on that, so it will be more than $24.99 but it should be less than $35.00 and I don't want to keep throwing money at his account.

Why does it insist I need to spend more money?

This does not seem like a good system for XBox users like me. Before I required him to ask permission I could just charge the needed amount to my credit card. Now I need to add money to his account in increments of $10 or $25 dollars and I can't figure out
how to spend the money, and when I figure it out there will be an odd amount of money left over sitting in his account that Microsoft gets to hold on to.

:)