How to make invoice header DFF not mandatory for certain type of invoice only, using Sandbox

Summary:

Hi,
We added a contextual DFF at invoice header level (in additional information tab). This DFF is used to select the department to which the request for approval will be sent.
This DFF is required and can't be empty. However, this selection must NOT be mandatory for Payment Requests invoice type, and we need to setup that rule so that invoice capturers don't need to navigate to that field and comply with the Dpt selection.
In other words, this DFF must be mandatory to complete for all invoice types BUT payment requests
We created a Sandbox using the page composer (see screen shots attached). we can't get to the expected result. Value is still required but there is now a "blank value" in the list of Values that looks considered as a relevant value and suffice to comply with the system requirement