A Microsoft offering that enables tracking of cloud usage and expenditures for Azure and other cloud providers.
It is usually applied automatically to your Azure subscription.
So, in your scenario, the system will automatically apply the $139 NZD credit, and you will be charged the remaining $61 NZD.
The difference should be payed from your credit card attached to subscription.
If you want to further clarify you can open a billing support request free here.
https://azure.microsoft.com/en-us/support/create-ticket
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Regards,
Abiola