Making a DFF mandatory depending on form field values

Hi

  We defined a DFF in Invoice Distributions in Payables to capture additional info about invoices. There are 2 requirements which need to br fulfilled

1. The DFF should be mandatory. i.e this will atleast make the users enter the values

2. The DFF should be mandatory for some defined values e.g. invoice type i.e.

Mandatory required for invoive type =standard/dr memo/cr memo

Not reqd for invoice type=expense report etc

How can this be achieved

Thanks

Ninad