money transfer agent

Refer to Rule 6 of Determination of Value of Taxable services under GST as per which

Determination of value in respect of certain supplies

(1)-

(2) The value of supply of services in relation to purchase or sale of foreign currency, including money changing, shall be determined by the supplier of service in the following manner: -

(a) For a currency, when exchanged from, or to, Indian Rupees (INR), the value shall be equal to the difference in the buying rate or the selling rate, as the case may be, and the Reserve Bank of India (RBI) reference rate for that currency at that time, multiplied by the total units of currency:

Thus broadly speaking the tax will be on margins.