Invoice Approval flow on Net Amount instead of Gross Amount

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Hi,

For the Approval Workflow of AP Invoices we've always used the attribute 'Invoice Header.Invoice Amount'. This is the gross amount, including the tax amount.

Now I need to evaluate the rules based on net amount (so without any tax).

I found the 'Base Amount' and 'Release Amount Net of Tax' but both give me errors while trying (because object is NULL?).

A formula does work: (Invoice Header.Invoice Amount-Invoice Header.Total Tax Amount)

However, that is not configurable through the Workflow Spreadsheet upload, so I prefer to use the actual attributes available.

Anyone any idea why the above mentioned (surely 'Base Amount') does not actually work? Are they used for another purpose perhaps?