NO Exchange rate difference between Invoice and Down Payment

Hello,

I want to post an invoice using the MIRO and reducing the total amount with the downpayment that I've already done. They are both in exchange currency (invoice and downpayment), but with different exchange rates, because they are created with different dates.

When I save, the MIRO generates a document without exchange difference rates because it put automatically the same exchange of the invoice to the downpayment.

In this case the amount in local currency reduced to the invoice is not correct, because I've paid a different amount in the downpayment. Below an example.

Local Currency: EUR

1. In date 03/01/2016 I make a downpayment:

Downpayment: 1000USD with rate 0,92 (920EUR)

2. In date 25/01/2016 I receive the invoice:

Invoice: 2000USD with rate 0,90 (1800EUR)

3. I create the invoice with the MIRO reducing the amount of the DP:

Debit: 2000USD with rate 0,90 (1800EUR)

Credit: 1000USD (I can't select the rate)

==> In this case SAP put to the downpayment the same rate of the invoice thus the amount of the invoice will be the following:

Debit: 2000USD  (1800EUR)

Credit: 1000USD (900EUR)

But it's wrong, I think it should be the following:

Debit: 2000USD (1800EUR)

Debit: 0 USD (20EUR)          -->  exchange rate differences

Credit:1000USD (920EUR)

Do you know how can we solve the problem?

Please let me know any idea.

Thanks a lot!

Regards

Matteo