Younium will always invoice up until the effective end date. When canceling an order what you effectively are doing is setting or confirming the order end date.
If the charges have been invoiced up until the former end date and you set a new end date before the previous end date the customer will have paid for more then you have delivered and a credit invoice for the residual will be generated.
Hence if you get a credit invoice when canceling the order, check how much of the order has all ready been invoiced.