It is possible to modify the stay dates of some rooms when multiple rooms are booked with the same reservation. However, the selectable dates are constrained by the booking global check-in and check-out dates. This function is helpful in case the guests of a room will arrive some days later or will leave some days before. It is important to note that the sole goal of this function is to free up the availability of some rooms before or after the booking check-out or check-in dates. Defining a lower number of nights of stay for a room will NOT affect the pricing, which will still be based on the total number of nights of stay of the main reservation. If you wish to adjust the pricing when a room needs to change the dates of stay, then you should remove this room from the main reservation and manually create a new booking for this room, for the same customer, by selecting the proper check-in and check-out dates. You basically need to move the room from one reservation to another.