I see a bunch of threads on reddit about startups accidentally going way over budget and then asking for credits back.
This doesn't at all mean the startups have bad intent, but things happen and Google doesn't want to deal with a huge collection issue.
If someone rolled up to your gas station and wanted to pump 10,000 gallons of gas but only pay you next month - would you allow it?
https://docs.cloud.google.com/apis/docs/capping-api-usage
Or you can do it programmatically.
https://docs.cloud.google.com/billing/docs/how-to/disable-bi...