Just edit the Amount Currency on the Journal Item representing the deposit side of the transaction.

In this example, your bank gave you $CAD 1,321.78 but the rate in Odoo will book it as $CAD 1,343.69.

Transfer:

Creates these GL Entries:

And affects your Bank Balances like this:

If you edit the Amount Currency on the Journal Item:

You will then see this when you go to reconcile:

Bank Balance after reconcilation: