How to make the Billing extension to override the defined rate schedules

Hi,

By using the billing extension, i am able to generate the events at top-task level and also generating the draft invoice/revenue amounts.
But the problem is, if any bill rates are defined for the expenditure item means, draft invoice/revenue amount is duplicating.
It is the sum of my calculations in my billing extension and the calculations of the (expenditure item * bill rate).
My billing extension is not able to override the defined rate schedules. i need to generate the draft revenue/invoice amounts with my billing extension calculations only.
Please let me know, how should i make my billing extension to override the defined rate schedules.