You can use GGB0 -FI Validations for your requirement for the fields BLDAT, BUDAT. I am not sure what the field ZFBDT is and I dont think thats a Standard SAP filed name.
You need to go to Financial Accounting- Document header and create a Validation rule as per your requirement. You can as well use an user exit in this functionality.
thanks to your answers so far.
Regards the validation and substitution functionality I still have a question. I have a Z-table where you can enter a date. This date is meant to be the lowest possible posting (BUDAT), document (BLDAT), base date (ZFBDT) the user can enter during an FB01 posting. How do I define a rule which compares the entered document date with the date maintained in my Z-table? And how do I trigger an appropriate (error) message?