User exit for VBo2

Hi friends

I need to find a userexit for Tcode "VBO2 - Change Rebate Agreement". The client wants to call tcode "FB75 - Enter Customer Credit memo" from that screen. We are using the version 4.6C, I am unable to find any userexit. The client wants the functionality at any cost can we do it from an access key if no user exit is there.

Thanks in advance