An over-booking based dynamic spectrum management (DSM) scheme is conceived for improving the attainable spectral efficiency. All secondary users (SU) will be categorized into different classes and they borrow spectral resource from the primary users (PU) before data transmission. Under the risk-based policy model, the effects of both booking cancellations and 'no-show' reservations are analyzed. Assuming that the booking demands obey an inhomogeneous Poisson process, we derive the optimal number of excess reservations, while minimizing the total compensation costs. Algorithms are developed for determining the capacity allocation dedicated to each class, whilst denying allocation, which would lead to congested bookings.