It seems that it's possible to have a spender with negative balance or even incorrect balance. The whitepaper mentions that the sender is responsible for writing the balance in the spent block. Furthermore, aggressive pruning is suggested to remove spent blocks. This means the full ledger is not available to fully verify.What prevents the sender from sending 10 coins though only subtracting 1 coin from their balance?
https://steemit.com/raiblocks/@selfdrivingsandp/negative-balance-in-raiblocks