Thanks Ajay. I did change the values in the User Exit you had mentioned, but it somehow didnt reflect in the Accounting part of the invoice.
But I found a solution for this through OBBH substitution, by making use of "only Exit" option, and wrote the code in my User exit. I substituted BSEG-PRCTR(Profit Center), based on BSEG-HKONT(G/L Account). This worked perfectly for me.