If you want to fix the exchange rate on a certain date, use the function GOOGLEFINANCE
(code [attribute]; [dataname]; [datacontainer|days]; [interval]
)Information about the function in Russian
For example, in A1 write (instead of USD and RUB, you can substitute any other currency):
=GoogleFinance("CURRENCY:USDRUB"; "close"; D2)
, write the date of purchase. And the currency pulled up from a certain date. Since this is historical data, they will not change.