Validation when posting a document using T. Code FB50 or F-02

Dear Experts

Please let me know on how to validate when posting a Cross Company Transaction using T. Codes FB50 or F-02. The requirement is to restrict Branches (each Branch is a Company Code in SAP) to post any Cross Company documents by selecting HO Company Code in Line Item, system has to stop the User by giving Error Message.

We can't control this using User level Authorizations because, here the Authorization is provided to the Users to post the Transactions using T. Codes FB50 and F-02.

To restrict this, I created a Validation as below:

Prerequisite:

BKPF-TCODE = 'FB50' OR BKPF-TCODE = 'F-02' AND

BKPF-BUKRS = 'Br. Company Code' AND

BSEG-BUKRS = 'HO Company Code'

Check:

FALSE

Message:

You Can't post Cross Company Transactions using T.Code FB50 or F-02

And I activated the above Validation in OB28 for HO and Branch Company Codes. But the Validation is not working properly. Can anybody help me on how to proceed further in this regard please.