As per Rule 34 of CGST Rules 2017, Foreign Exchange Rate for Determination of Value, The rate of exchange for determination of value of taxable services shall be the applicable rate of exchange determined as per the generally accepted accounting principles for the date of time of supply of such services in terms of section 13 of the Act. In simple words, the rate of exchange prevailing on the date of supply sahll be the rate of initial recognition in INR.
Later on, if the payment is received in the same F.Y. then you can enter the amount so recieved in the financial statements of current year. If however payment is recieved in next financial year, then on closing the financial statements that receivable will be recorded at the closing rate and subsequently when payment is recieved the exchange difference arising out of it shall be debited or credited to P&L A/c as the case may be.
The above treatment is as per the FRF applicable to your entity.
Yes, you will have to mention the type of export (example: export under LUT) in the invoice that you may prepare for clients outside india.
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Shiv kumar Agarwal