Can I express 100 as a three-digit 9's complement number?
Nine's complement representation involves a choice of the exact range of positive and negative numbers. It is natural to let the sign depend on the MSD (most significant digit). It is even more natural to declare that numbers starting with $0,1,2,3,4$ are positive, those starting with $5,6,7,8,9$ are negative. The range of three-digit numbers is then $-500$ to $499$ rather than $-900$ to $99$, and $100$ becomes representable. I suspect that this interpretation is what Tanenbaum intended.