GST liabilty on unregistered person - Goods and Services Tax - GST

My client is a mutual fund broker. Since the inception of GST, the mutual companies give brokerage inclusive of GST irrespective whether the broker is registered or unregistered under GST. In 2018-19, my client received brokerage of Rs. 9,50,000 which also includes GST. As per GST law, GST registration was not required as turnover is less than 20 lakhs. Now GST department is asking my client to pay the GST received from mutual fund companies for 2018-19 even there was not registration requirement at that time. Whether we need to pay the GST or not Request the experts to provide your views on the above issue <!--/data/user/0/com.samsung.android.app.notes/files/clipdata/clipdata_bodytext_240724_130350_047.sdocx-->