It would be a nice ehancement to adjust 'Exchange rate' for Payment if invoice has been posted in a different currency.

The business scenario is as follows:
a. Accounting currency MXN
b. Exchange rate 20 MXN = 1 USD
c. You post a 100 USD Sales order. It would be 2,000 MXN (20 * 100).
d. You agree an Exchange rate with customer 15 MXN = 1 USD
e. So, you will want to use Fixed exchange rate in Sales order's Header. You will invoice 1,500 MXN instead of 2,000 MXN because the agreement with customer.
f. Then you get payment in MXN. Since Accounting currency and Payment currency are the same, there is no option to update the 'Exchange rate'. So, when you get payment, you will collect 2,000 MXN (100 USD by Exchange rate which is 20 MXN = 1 USD)

Unfortunatelly, the Exchange rate agreed with customer is overriden.

Note. Associated scenarios like: Discount and Conditional sales tax, should be included.

This is called spot exchange rate.