very very very urgent

as per the new point of taxation rules, the building contractor has to determine the pt. of taxation based on either receipt of money from the client or invoice date whichever is earlier.my question is on what amount service tax should be calculated in case the taxable event is raising of bill or completion of service, as it is not certain that the client will pay the full bill amount.there might be some retention of money as security deposite which may or may not be recoverable.in that case the tax will be paid out of our pocket. pls clearify this issue.