Can billing account viewer approve invoices?
It can only "view billing account cost information and transactions".
According to the "Billing Access Control" video, they can (around 8:35 time mark). I imagine that seeing the bill is sufficient for you to send them the proper amount of money? Link to documentation on what an invoice looks like. https://cloud.google.com/billing/docs/how-to/read-invoice
I believe the invoice approval is not done within GCP, but it’s more of a company internal action. Obviously, to be able to process invoices for your company, you need to be able to see them.
In the documentation https://cloud.google.com/billing/docs/how-to/billing-access there is explicitly written among the examples. In the Scenario "Separate financial planning & procurement functions"
User: Account Payable
Role:Billing Account Viewer
Activity: Approve invoices