Hi Fernando Luna,
I doubt about validation too.
I would try one of following 2 BTEs. Both are getting called right after you hit enter on changed field in FB02.
P&S module : 00001136 CHANGE DOCUMENT: Exclude Field Changes
Process interface: 00001420 CHANGE DOCUMENT: Field modification to line item
In ABAP code check if we are in FB02, compare say I_BSEG-ZTERM with BSEG-ZTERM and if different then it’s your “case” 
Best approach in ABAP code is to do AUTHORITY-CHECK on custom authorization object. This Z-object you assign via a role to all authorized persons so everyone else will be rejected. Such kind of object could even be included in SU24 for t-code FB02, just to make sure every role in the future gets it.
Another easier way around is to create a custom table containing a list of authorized users or check in code for SY-UNAME against Z-table. This second option is slightly worse because afterwards we usually forget to maintain such kind of table when personnel turnaround happens.
Hope you like this idea.
Best regards,
Anton