Field exit for field AFPOD-CHARG

Hi all,

I'm sorry to bother you with this subject, but, I am struggling to develop a field exit for field AFPOD-CHARG but I am not able to...

I have a user requirement demanding to block that field after filling it and releasing the Process Order (tcode COR1 and/or COR2).

I don't know why, INPUT comes equal to OUTPUT from the beginning and I can't check if the user changed the field or not...

Could anybody help me up?

Many thanks in advace,

Alberto