Currencies in payments

[Maja Damber]

We are doing an integration for a customer and I noticed that the structure does not seem to be the same for customer payments and supplier payments when it comes to payments in other currencies than base. 

For supplier invoices the paid amount is in base currency even though the currency on the entry is something else. 

But for customer invoices the paid amount is in the currency the invoice is in. 

Can I assume that this is how it always is or is it due to our customer doing the payment in a way that triggers another handling of  the currency? 

We will be using this integration for other customers as well so we need to know that it will work in all situations.