service tax - pure agent

Dear Sirs,
We are purchasing recharge coupons from uninor & sale the same through retailers. WE got retail commission from uninor after deducting income tax TDS & we pay the net amount i.e. after deducting TDS to retailers.
Whether we have to collect service tax on the whole retail commission or the net amount (difference between retail commission received from Uninor & retail commission paid to retailers). Whether we can claim retail commission paid as pure agent service