How to return ticket to on-account fund?

What's the best way to return a ticket and put the order credit into an on-account fund (via the web API)?

I've tried loading a ticket's order into the cart and then calling ReturnTicket(), but it always throws an exception, "Invalid seat_no/checksum data".

Thanks,
Bryan

[edited by: Bryan Drenner at 2:15 PM (GMT -6) on 2 Nov 2011]