How to Restrict system not to accept project costs, invoices when task is still chargeable

Summary:

We have created some projects with a custom project status called "Preliminary Authorization". We also have a DFF at the task level called "Preliminary Flag". The business requirement is that when the project status is "Preliminary Authorization", and the financial task is marked as Chargeable but the Preliminary Flag is set to 'N', then no project costs or invoices should be allowed to be booked against that task. Please suggest how to fulfil this Requirement

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