Dear Sir,
Form 15CB is for reporting purpose. There might be difference between the exchange rate used in Form 15CB and rate used at the time of actual payment. You should report the actual payment and TDS deduction in the TDS return.
In the case for RCM paid on import of goods/services. There might be difference between the exchange rate used in the invoice and actual date of payment. You need to pay RCM on the basis of Invoice only.
Thanks