For version 14, this is managed via the Unrealized Currency Gains / Losses Report – which will automatically calculate the unrealized gain / loss and propose a journal entry (and reversal) based on comparing the current exchange rate with the rates used to book each transaction:

Learn more about the Unrealized Currency Gains / Losses Report at http://www.youtube.com/watch?v=z_ADEpQdtUo&t=21m9s

For version 13 and v14, the Consolidation App allows you to present adjusted financials without needing to record (or reverse) journal entries. 

Scenario:

Your Company maintains a General Ledger in USD, and you have $25,000 in a US Bank Account and no other assets and/or liabilities.

November 1^st, you purchase 20,000 € at a rate of 1.1, meaning you spent $USD 22,000.

At the end of the month, the rate is 1.2, meaning they are worth $USD 24,000 if sold, so you wish to generate an adjusted Balance Sheet to reflect this.

Since Odoo records all transactions in the functional currency of each Company at the rate in effect on the date of the transaction, your November 1^st and November 30^th Balance Sheet will look like this:

You can build a Consolidated report for any period you wish, monthly in this case, and record the potential gain/loss on any foreign currency holdings via consolidation entries (that do not affect the general ledger).

Before adjustments, your Consolidated Balance Sheet (of a single Company) would look something like this:

You then edit this balance and add an Adjustments column.  

Drill down to create this Adjustment Entry:

The Amount field can accept a formula – so here we have the difference between 1.1 (starting rate) and 1.2 (ending rate) over the ending rate multiplied by 20,000 (amount of Euros to adjust).

Balance your statement with a $2000 adjustment to FX Adjustments to get a Consolidated Balance Sheet like this:

The adjustment made for potential FX gains (1) and corresponding adjustment to Equity (2) result in a balanced statement showing the potential additional $USD 2,000 in Assets, being added to the existing $USD 3,000 held for a potential total asset balance of $USD 27,000.

At this point the Consolidated Balance sheet can be exported to Excel to add any other notations before being presented.

Learn more about the Consolidation App at https://www.youtube.com/watch?v=zwBexsSfxt8