Hi Fred,
I think, I know, what is your issue. I explain you, how you would do this.
Let me use the check as payment method to avoid additional steps.
Your local currency is USD.
You have a downpayment request over 150,-EUR (which equals in your system 195,-USD).
Now you receive a check over 167,-USD (as the exchange rate changes).
If you now assign the downpayment request to the incoming check, you see a difference of 28USD. For this difference you can select the payment difference reason "97 - Currency exchange differences".
Payment difference reason is only available, if the payment and the document currency are not equal.
Please let me know, if you have questions.
Best regards
Marlene