IAS 39 specifically deals with investments held in your 'trading book' or 'available for sale' book. Firstly, it is appropriate that all deals between affiliates be at market rates. Secondly, the bonds have to be 'marked to market' each month end. If they are in the trading book, the profit/loss is charged towards net income. If they are in the available for sale book, the profit/loss is charged towards shareholders equity.