Alternatively, won't the TGV stop at Massy TGV on the way round? In which case, just hop of the train at Massy (which is a good 50 minutes before CDG), and get the RER B into town (25 minutes or so). It'll barely take you longer than the direct train, particularly given the journey in from Montparnasse (unless that's where you're actually headed).
I wouldn't hold out any hope of getting your money back on an inflexible ticket.