GST on reverse charge basis - Goods and Services Tax - GST

One of our clients, a recognized educational institution and not registered under GST Act , has invited some guest professors from abroad to deliver lectures. The return airfare of the foreign professors would be reimbursed by the client, in foreign currency, at actuals. Is there any liability to pay IGST on reverse charge basis on the reimbursed amount? Experts opinion would be greatly appreciated.