Non-iata air travel agent

Client is a non IATA Agent...
Buys ticket for Customers
Eg:- (Issued a Ticket to Customer @ 20,000 on which Supplier allowed a discount of Rs.1000, Normally have to pay supplier Rs.19000, but supplier collects Service Tax on Discount allowed Rs.123. Hence Paid to Supplier Rs.19123)

Entry
Cr. Supplier -19130
Cr. Discount - 1000
Dr. Service Tax Paid -123
Dr. Customer -20000

Is this service tax paid to supplier allowed in Cenvat Credit 
Is Reverse Charge Mechanism apply here
Is we have to pay service tax again