Applicability of service tax on purchase rebate/incentives

Hi Friends,

I hope many of u know about the incentive/backend practive in software industry.

in this situation we are purchasing the software at Rs. 100/- and we are selling it  in Rs. 90/-. aftert that if we have purchased certain no. of units or certain amount we will receive cheque from software co. of Rs. 15/-. it means our profit is Rs. 5/-

now my question is " if my backend/incentive receipt is more than 10 lakhs we need to pay service tax or not ?

in my opinion it is a purchase rebate only on the basis of volume, what is ur view on it ? whether we need to collect service tax ?

please support ur answer with relevant proof or cases.