Cannot change son's gamertag

I want to change my son's gamertag. I therefore logged into xbox.com with his Microsoft account credentials and attempted to change the gamertag in his profile. I selected a new name, which was accepted.

This prompted an e-mail from Microsoft to my own address, requesting permission to change his gamertag. I responded in the affirmative and was told that there would be a fee of $9.99 to change the name.

I therefore loaded £10 GBP onto his account using my credit card. This sum is now showing correctly on his account as a credit (NB: £10 > $9.99).

Subsequently, I logged back into his account and attempted to change the gamertag again. The new name selected was again approved and claimed.

When it came to the payment option, I clicked on the default, which was to pay the fee from the account credit. However, this option failed, with the following message: "We’re having trouble processing your payment and we’d like to get it sorted out". I have been round this loop multiple times.

Options to speak to a person in Microsoft also fail. The assistance bots have been useless in solving the matter and just refer to the stock instructions for the process described above that I already tried to follow.

Any help, please?

:)