money exchange

If currency is exchanged from or to Indian Rupees (INR), then difference between the exchange rate (buying or selling) and RBI reference rate multiplied by the total units of currency should be the taxable value.
For example, Mr. A provides currency exchange service. He purchase 100 USD at the rate of 63, at the time of purchase RBI reference rate was 65. In this case taxable supply will be difference between RBI reference rate and actual purchase rate. In this case value of supply will be 100x(65-63) = 200 Rs.
If RBI reference rate is not available for the currency, then the value of taxable supply will be 1% of the gross amount of Indian Rupees provided or received.