So, I tried several things. Nothing was working! But then I realised that the DATE on the euro invoice was wrong. I adjusted it and now I get the same value. Great!
And I had to have the exchange rate. Without it, even with the inter account transfer, the cost of the item is not correct.
Now, going back to the initial question of this thread, I think it would help to have the possibility to add exchange rates both ways, that is to say in my case for example 1 RWF = 0,000990099 but also 1 EUR = 1.010 RWF. It would make things a lot easier, I think.
Thank you again!