This article explains what the Allow invoice modifications setting does, and how it works together with the Edit/credit/delete invoice permission.
The behavior controlled by Allow invoice modifications is not mapped one-to-one to a single permission. Some of it overlaps with the Edit/credit/delete invoice permission (which also covers more, such as crediting), while another part is not tied to any permission at all (such as choosing the invoice date when invoicing an older record).
What you need for each action
Editing or deleting an invoice: Both the Allow invoice modifications setting and the Edit/credit/delete invoice permission must be enabled.
Choosing the invoice date (when invoicing an older record): Only the Allow invoice modifications setting needs to be enabled.
Good to know
When Allow invoice modifications is turned off, no permissions are disabled or greyed out. This is expected behavior, because no permissions are linked one-to-one to this setting.
User role (Edit/credit/delete invoice)
