Hi Ankur,
If you want to create Instalment plan during IS-U Invoicing, please make sure the amount you are rolling into that Instalment plan is right and take extra care.
Please use R428 to achieve your requirement, for invoicing check parameter X_CALL_ID = I for actual Invoicing, then use FM FKK_S_INSTPLAN_CREATE to create Instalment plan.
But if you need specific FI-CA items which are included in Invoice (like in T_VKK_DOC_ID) then use R431 and R433.
Thanks,
Srini