Ok, so you only need to enter an exchange rate for the Invoice, as there is no need to enter an exchange rate for the payment. If your euro supplier sends you an invoice for 100 euros, then the Purchase Invoice would look like this using 1 RWF = 0.8 euros:
And the Summary tab would be this
If you need to spend 150 RWF to acquire the 100 euros then just enter both of those currency amounts in the Spend Money when paying the invoice:
Now the Summary tab looks like this:
Read this topic where there is a worked example of importing inventory with separate supplier invoice + courier/freight invoice + import duty/taxes and using Freight-in