If the customer or supplier is set to Any Currency. It will be possible to post a home currency adjustment to balance off the exchange adjustment postings.
Once this has been done, the transactions can be allocated against each other, in the home currency, via the allocation area.
To set the record as Any Currency:
Go to the Customer or Supplier record.
select the Terms tab
tick Any Currency (note the warning about not being able to undo this)
Save & Close
Note: As detailed above, setting a customer or supplier to Any Currency cannot be undone.