FI posting data Modification

Hai ,

I have a Requirement in FI when ever the posting is happening based on the GL it should pick the transaction type 120 as default . how can i achive this , I have tried with the bte 1020 posting interface the values are getting modified in the run time but finally the sap standard code is again modifying the data , some body can he me in this regard ASAP .

Regards,

K.Vinay Kumar