Dear Gloria,
You have to enter a "special" exchange rate.
Here's an example: 
the exchange rate between CAD and USD is normally: 1 CAD = 0.72 USD, 
or 50 CAD = 36 USD. 
However, if you use an exchange rate of 0.00000001, the program calculates 50 CAD = 0.0000001 USD, 
which is rounded to 0.00 USD. 
This doesn't change the USD balance, but it does debit the 50 CAD.

This allowed me to adjust my difference.
Best regards, 
Albert