Service tax short payment of bill

Hello everyone.we are a private ltd co and we hire security services for our site.they raise the bill adding their part of service tax i.e. 25% of total service tax under reverse charge mechanism. For the mnth of feb 2013 they have raised a bill of rs 137000 including st. But we are deducting rs 2500 for negligence in services. Are we required to pay service tax on rs 2500 that we are deducting?what are the other ways?the bill doesnt have any mention of rs 2500.it is gross.