migo date control

Dear experts,

I want to put control in migo screen.

po date is 25.11.2011

Issue date is 26.11.2011 (mb1b tcode, 351 movement type)

but gr date is 25.11.2011 (migo, 101 mtype )

at present system accepting this entry

so, I want to make a control on migo screen like gr date should be greattr than or equall to issue date.

regards,

Ali