I think you were right about the posting date being changed.
This is probably what happened:
• The invoice was entered by accident with previous date
• The date was corrected to the right date
• The first popup appeared mentioning “You have changed….”
• Then the second popup "Do you want to update the exchange rate" appeared. (This popup is often not expected)
• The OK button of the first popup is at the same position as the “No” button of the second popup.
• So if the user was in a hurry he probably clicked twice at the same cursor position and accidently said “no” to update the exchange rate.