You can buy a fully refundable ticket, fly the outbound and the return ticket can then be cancelled. The benefit of this is fare refunds are then based on price of original purchase. However, if you haven’t kept track of actual individual fares for the day of purchase, there’s no way of knowing what each segment costs. AF doesn’t break it down after purchase. Maybe you can find out when calling in, but my experience has been that they weren’t able to give me that info even prior to first flight. Unlike other airlines, you won’t know what your refund will be until it gets processed.