A Microsoft offering that enables tracking of cloud usage and expenditures for Azure and other cloud providers.
I know this is an old unanswered Q - but it appeared in my searches for this exact problem so I'll reawaken it.
I progressed a Q of my own: https://learn.microsoft.com/en-us/answers/questions/1861859/how-to-add-invoice-manager-role-iam
Solution is that the Invoice Manager role is only available in IAM for the "Cost Management + Billing -- Billing profiles blade"
Important note: this is not the cost management + billing section on the standard subscription view blade, and you can't get to the right IAM section from there. This was the source of my confusion.
You can get to the right "blade" via the main burger menu "Cost Management + Billing".
https://portal.azure.com/#view/Microsoft_Azure_GTM/ModernBillingMenuBlade/~/BillingProfiles
Note - I still don't know what a blade is as a UX concept!