mandatory custon field in me21n based on gross value

Hi ,

  i added custem field in me21n header level .. i want to make that field mandatory only when gross value greater than some amount...

  i done upto addition of custem field ..

i'm not getting gross value amount in EXit_SAPMM06E_006  or EXIT_SAPMM06E_007...

guide me how to get gross value amount ..

regards,

Anbu