Hi
It should be a simple validation at line item level
Prerequisite:
BSEG-HKONT=1000
Check
BSEG-KOSTL=100XX
Messge:
The cost center is not valid for GL Account 1000
The below link should help you how to configure a validation rule in FI
Validations - ERP Financials - SCN Wiki
Regards
Sanil Bhandari