Hi Ranjeet,
For your question Yes, The system will always pick the latest values maintained for the Currency in OB08. The actual exchange rate calculation posted as Exchange gain/loss in P&L acocunt and the Vendor accounts respectively.
However, You absolutely can enter manually exchange rate which is same as ex rate at invoice time while making the payment. (The field is next to Currency field)
As per system standard behavior, if you don't fill exchange rate the system will pick up the rate at the payment time, not at invoice time.