UserExit or BADI for Posted FI document change in FB02

All Gurus,

Can anybody tell if there exists any User exit/BADI that can be used for screen 750 in transaction FB02 that is used for a Posted FI document change.

Our requirement is to prevent user from making any changes to a Posted FI document of type Down Payment Request (with Status BKPF-BSTAT = 'S' - Note Items) via FB02. For similar requirement on a Parked FI document (Status = 'V'), I was able to find a std. FORM "BELEG_ANZEIGEN" that i could implicitly enhance to prevent the user from making any changes to Parked document via FB02 / FBV2 or FBV0. Even though both Parked and Posted FI documents go thru' the initial screen of 100, the succeeding screen / program (Screen # - 0750) for Posted Down Payment Request document of status 'S' is different from that of other Posted documents or a Parked document.

Has anybody had similar requirement and resolved it?. If yes or if you have any idea on how to achieve this, please reply with details. I understand we can use BTE's (1110 for example) while Saving the Posted Document Changes, but the user requirement is to prevent the user from making the changes in the first place rather than allowing the user to make changes and then validate these when the corresponding BTE is triggered.

We are using SAP ECC 6.0 with

Thanks in advance..

Venu