Hello @Birgit84 ,
If the guest booked allready (confirmed reservation), then you can not sent a special offer for the same dates, nor should it be used for an extension. You had to change the reservation to extend it (and offcourse this "extension-proposal' can be a "special offer" if you make it a special price). So your posting is somehow confusing. But the original dates your guest will stay will still be on the original confirmed reservation (see : list of reservations, print reservation). Also have a look at your multi-calender and in your dashboard (scroll down) to see upcoming check-in's/out's
Best regards, Emiel