Hi,
You cannot use your credits to pay for third-party offerings.
For some Azure account types you have the ability to disable purchasing paid marketplace offers via the billing profile. To check you would navigate to Cost Management + Billing -- Billing -- Billing profiles, click on the billing profile.
Once on the billing profile screen you would click Settings -- Policies, then under Azure marketplace purchases select Free or No and click Save.
Article below has more details:
Purchase control options in Azure Marketplace
https://learn.microsoft.com/en-us/marketplace/purchase-control-options
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Thanks.
-TP