Clarity about service tax treatment

X & Co. is a Distribution Agent . they are raising invoices including service tax but agent refuses to pay service tax stating he has already paid it to government & paid amount without including service tax. ( eg. Invoice 1000+ st 103= 1103. he  receives only 1000 )  what will be the service tax treatment in books of X & co.?  has he have to back calculate service  tax  assuming 1000 is inclusive of service tax ? or he have to take proofs about already paid service tax ?