Hi Nazrul,
Sorry but was my mistake, please try with the function: /IBS/RB_CML_EFFKAPITAL_GET
Or maybe you can try with the function: BAPI_LOAN_GETDETAIL_MLT, with this FM, you can view all the cash flow of the loan, and you can select only the flow types that are into your commitment.
Another variant to calculate the commitment of a loan, you should try to look into the tables VDBEPP for a specific Payment Date (DDISPO) and verify the field BBASIS (Calculation base amount)
I hope it's useful for you,
Regards,
Ivan