not sure why its not working, it should be as rate_update basically calls _credit_update(), perhaps if you can share your modification I can examine, and check logs too
as for negative balance see this:
I added addhoc balance (temp balance) and use that to determine if user can send next SMS or not
the adhoc balance is necessary because the way playSMS bills, it needs to wait for status of each SMS become sent or failed, and since SMS is alot then it takes time
if you found better way let us know
anton