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I would like to void a pending Invoice before closing it first. Would this be possible?

Solution

Invoices for subscriptions with metered components are automatically generated in the pending state which means that till now the invoices are in the draft state and in the process of getting generated.

So until you add the relevant usage and close the pending(draft) invoice, the invoice is not actually considered as generated and this is the reason why the void option is not available while the Invoice is in Pending Status.

Related Article:
Regenerate invoice error - "There were errors while submitting"

What are the limitations of voiding and regenerating an invoice?