How can I get the exchange rate applied to specific transactions from the API

I'm trying to build the following logic:

1. End user makes a purchase/sub
2. We track that with transaction ID
3. We review the API (?) to get the exchange rate that is applied to that specific transaction to figure out the value we receive from it in our currency
4. We Aggregate across certain transaction metrics (eg. product, day, etc.) to calculate the actual value received in our currency in a given period (month/week/etc.

Step 3 is just not working. I'm hitting the financial api and the sales api, but there doesn't seem to be sufficient data to identify this?