Have you tried the accruals functionality in SAP , which specifically provided room for dealing such processes . Where in you create an accrual object and then run the accural run per month to have the nullification entry in your books .
ACACTREE01 - You create accrual object .
ACACACT - Priodic run .
Ofcourse it needs some baseline in place to start using it .
Hope this helps.
Regards.