Hi Vijay,
If you are using the Service notifications in SAP, then retrieve the object number (VIQMEL-OBJNR) against the service Notification (VIQMEL-QMNUM).
Check the field JEST-STAT EQ "E0010" and field JEST-INACT EQ Blank by putting the object number retrieved above in the field JEST-OBJNR.
If the condition satisfies, then the installation is disconnected.
Now you can use the installation to find the contract and the contract account through FM ISU_GET_OBJECTS.
Once you find the CA/C, you can use the FM FKK_ACCOUNT_BALANCE_COMPUTE to find out the outstanding balance.
Or, if you only want the amount for which the disconnection has happened, then you query table FKKMAKO\FKKMAZE to find the amount outstanding for a particular dunning level for which disconnection has been triggered.
Hope it helps...
Thanks,
Amlan