How can DFF become mandatory basis on selection of a particular GL code?

We have requirement that if Maker 1 while creating an invoice selects Electricity Expenses gl code and forgets to fill in Unit/Rate which is a line level DFF and initiates this invoice for approval it should get auto rejected giving the reason that Unit/Rate DFF is blank. Similarly if Maker 1 selects Motor Car expenses (Fuel) gl code and forgets to fill in Type,Rate and Quantity which are line level DFF's and initiates this invoice for approval it should get auto rejected giving the reason that these DFF's is not filled.